Hello, I checked the guide section and saw how empty it was, so I figured I would try and make a simple guide for the 1 thing I think this game needs a guide for.

The Tablet challenges.

These are special challenges for levels that unlock after beating a level once, if you replay a level and beat it while doing what is shown on the tablet + avoid losing more then 50% of your lives (kids), you'll earn an extra skill point.

Each Game mode has their own tablet challenges, so that's 3 extra skill points you can earn from each level if you complete them all.

And if you are a completionist, you were gonna do them anyway for that 100% feeling.

Random info before starting:

Important note - I did (almost) all the challenges on Normal Difficulty, as the Hard and Insane difficulties do not give any additional rewards and are solely for players who want an increased challenge/bragging rights.
So any tips I suggest might not work as well/at all if you are playing on them

Hot keys - to avoid having to click on everything.
1,2,3 - Switches between normal,fast,super fast speeds.
Q,W,E,R,T - Hot keys for your Leader abilities (AKA spells).
A,S,D,F - Hot keys for building/upgrading towers.
Space - Toggles super slow mode. You can't build/cast during this, so it's only for checking enemy stats.
Enter - Spawns the next wave if the bubble is on the screen.

Also, this happens to be my first guide, so it's probably a bit rough in parts.

• Level 1 - Complete the Level in under 4 minutes.
  Tips: Start the waves as soon as you can. The Enter Key can start them early as well.
  
• Level 2 - Finish the level building 2 or less Spearman.
  Tips: Have some upgrades under your belt, and use spikes on enemies that resist Blunt/Magic damage.
  
• Level 3 - Have 13 or more units with the Level 2 Keeper's Range buff on them at the same time.
  Tips: Get the upgrade that increases the buff's duration, and build enough level 2 Keepers where they can reach 2-3 units each.
  
• Level 4 - Have 4 or more Stone barricades from Colossus's unlockable ability on the map at once.
  Tips: Unlock the Colossus's Stone Barricade skill 5 skills deep in the Stoneman section of the skill-tree, then get 2-3 Colossus's with the skill till you get enough Stone Barricades on the map.
  *Bug fixed. Thank the Devs!*
  
• Level 5 - Complete the Level without using the Boulder Launch spell.
  Tips: You'll want some well placed Stoneman with decent upgrades to deal with swarms that you can't chuck boulders at.
  
• Level 6 - Complete the Level in under 27 minutes.
  Tips: Start waves fast enough, and remember the boss's spawn locations/order and make sure you have the right units placed nearby to shoot it down fast enough. Spearman and Shaman upgrades help.
  
• Level 7 - Beat the Level with 2 or more of the Big Bone Barricades still intact.
  Tips: Use your spells on the big exploding lizards? to kill them before they get near the later barricades. Lots of Spearman with upgrades also help with that.
  
• Level 8 - Beat the Level without building any Colossus's.
  Tips: You can still build Beasts though, 2-3 of them in good spots with upgrades can easily make up for your lack of Colossus's.
  
• Level 9 - Have 30 or more units under the Dino Keeper's Damage aura at the same time.
  Tips: Get the upgrade that increases the Radius of the damage aura, then just place enough Dino Keepers to buff enough units.
  
• Level 10 - Beat the level building 3 or less Keepers.
  Tips: Get the upgrade that increases the food keepers produce, then place 1 Keeper per lane. You can still upgrade them, so just build a bunch of high level towers around them instead of spreading towers out.
  
• Level 11 - Beat the Level without using Spike Traps.
  Tips: Since you can't catch leaks or weaken big enemies with Spikes, Have enough upgrades so your towers+other spells can handle them instead.
  
• Level 12 - Warp back 151 or more enemies with the Magic Circles
  Tips: To activate the magic circles, wait till they start sparking with blue energy, then click the 'lid' on the bottom circle. They will warp back any enemy that enters or exits a circle while active, so use it right before a swarm of enemies passes over a circle for best results.
  If needed, you can try and stall during the boss to get to 151 if you somehow don't have enough by the time you reach the boss
  
• Level 13 - Beat the Level without building any Ice Shamans.
  Tips: Since Ice Shamans are the only tower that resist the freezing effect this Level has, you'll need to spread your defenses evenly around the fire-pits so you always have at least 1 section per lane warmed up and able to defend.
  
• Level 14 - Have the Ambulocetus Devour 500,000 HP's worth of enemies.
  Tips: If you haven't already found it yet, this level's secret thing is the frozen monster in the top left.
  Stick a level 3 or 4 berserker on the suspiciously placed X mark next to it and wait for it to break the monster free. After that, hold your mouse over the monster's head to see its target area, then click to make it chomp down on the enemies on that spot, dealing massive damage or outright eating whole any enemies in area. After which it goes into a recharging animation of it chewing for a while before being ready again. So putting walls in front of its 'chomp' area to clump enemies up before chomping is recommended. And waiting as long as possible between waves also helps a bit.
  (Praise the Devs! the challenge got lowered to a sane amount!)
  Anyway, for tips, I would build 90% of your defenses in the final section behind the Ambu, with 1-2 shamans/fish keepers elsewhere to slow/stall. And for when to chomp with the Ambu, try focusing on the larger enemies when you can, though for some waves you can take several smaller chomps instead. Also, since higher difficulties raise enemy HP, you can try doing the mission on those if you can handle the waves to speed up your progress a bit. When I did it on impossible, I hit the target goal at the start of wave 24 chomping on some of the big woolly mammoths.
  My only warning would be beware of the flying moose things, as they can fly to the exit in large numbers depending on how your towers are placed, so they might leak enough to cost you the <50% lives lost part of the challenge if you get careless.
  
• Level 15 - Beat the Level without snow falling on any Towers inside the fenced 'snowfall' area
  Tips: Unless someone knows the exact reason snow falls on towers inside the fenced area, I'm just guessing the only way to do this challenge is to avoid building any towers inside the fenced area.
  Make sure you fortify the left side enough to handle the boss.

• Level 1 - Do not use the Boulder Toss or Barricade spells
  Tips: Enough upgrades makes this easy enough.
  
• Level 2 - Win with at last 39 of the pre-placed Barricades still intact
  Tips: Enough upgrades makes this easy enough.
  
• Level 3 - Win in less then 16 minutes Complete the mission without using Spikes or Spear Defender Level #2.
  Tips: Previous challenge was bugged, it got changed to the new challenge. Seems simple enough.
  
• Level 4 - Do not build any Blood Shamans or Level 4 Ice Shamans
  Tips: Since this level's gimmick is you only have the pre-placed Shamans as towers, but super fast spell cooldowns, feel free to re-spec your skills to focus on those and just keep throwing them at the enemies. If you got enough upgrades it's pretty easy
  
• Level 5 - Have 14 or more of the death weeds spawned from a Level 4 Colossus's ultimate ability alive at once
  Tips:
  
• Level 6 - Do not use the Barricade spell.
  Tips: Enough upgrades makes this easy enough.
  
• Level 7 - Win building only a maximum of 2 towers of each type of level 3 tower?
  Tips: I'm still not 100% sure, but I only built 2 of each level 3 tower to be safe, and I got it.
  
• Level 8 - Transform 5 enemies into snails at the same time with the Blood Shaman's poly-morph ability
  Tips: It got fixed so that it's now at the same time, I'm guessing you might want to try saving up enough food to upgrade 5 blood shamans with the hex ability all at once with they all have enemies in range to do the challenge.
  
• Level 9 - Have Bird keepers cast their speed buff on 24 or more towers at the same time.
  Tips: Anyway, just make sure you got the upgrades boosting the Bird keeper's buffs, then just build enough bird keepers and a bunch of basic Spearman to receive those buffs and around wave 9/10 is when I got the challenge complete check-mark.
  
• Level 10 - Do not use the horror spell or build Blood Shamans
  Tips: Beasts with both abilities can handle the swarms, and Berserkers can handle the bigger enemies. You'll want Shamans on the middle/lower path for UFOs though
  
• Level 11 - Don't build any more Keepers
  Tips: Sticking a Level 3 Beast with both abilities right under the bottom spawn at the start helps deal with tons of spawns from there. Also, put a Colossus on the Right exit and for wave 2 a Berserker on the Left exit and they can handle the leaks from the early waves.
  Just be careful of the big exploding lizards on the west, and the screech bats that spawn from the exits.
  
• Level 12 - Do not use the Boulder Toss, Barricade, or Horror spells.
  Tips: You can still use spikes and the rage totem spell at least, and you start with tons of food.
  So I put 2 Colossus's per side near the exit with stone barricade to make up for the lack of the barricade spell, and put a bunch of various high level towers in good spots to deal with all the enemies. I also think the warp circle cool-down is lower in this mode as well, letting you use it more often. Not 100% sure though.
  Also, 1 final warning for the end. A huge swarm of the flying screechers will come at you from the Bottom 2 paths, and a smaller amount of them from the upper paths. So make sure you got some towers in place and spread out to handle them and their death stuns, lest they stun-lock their way through your defenses and make you lose more the 50% of your lives.
  
• Level 13 - Win building only a maximum of 4 towers of each type of level 3 tower?
  Tips: I'm still not 100% sure, but I only built 4 of each level 3 tower to be safe, and I got it.
  
• Level 14 - Don't build any Berserkers.
  Tips: Without Berserkers and their piercing damage DPS, you'll need to adjust your kill zone towers to deal with yeti swarms without them. Then a couple of level 4 Hunters with flame spears can handle the big tough enemies. Blood shaman's poly-morph ability can also help deal with those if you get lucky with their aim.
  Also, make sure to build enough towers to soak/kill the flying planes from the left, and later the flying mutant enemies from both the left and right. As you need to prevent enough leaks to stay above 50% lives.
  
• Level 15 - Win with at least 11 of the big stone barricades still standing
  Tips: Good news, the fenced area does not have any snowfall in this mode, so go ahead and make a huge killzone in the starting area.
  Make sure you get some Spearman and Shamans near the exit to catch the flying enemies though, and later on in the bottom right section to handle the load of planes that come in later waves.

• Level 1 - You can only build 2 levels worth of Stonemen
  Tips: Because you can only build 2 Stonemen or one level 2 Stoneman, you'll need to build a bunch of spread-out Spearman to deal with the blobs. Use Shamans, spikes, and level 2 keepers to deal with the bigger bugs and clams.
  Using walls to clump up swarms and tossing a boulder on them also helps.
  
• Level 2 - Win building only a maximum of 1 tower of each type of level 3 tower?
  Tips: I'm still not 100% sure, but I only built 1 of each level 3 tower to be safe, and I got it.
  
• Level 3 - Beat the Level with 3 or more of the large stone barricades still standing.
  Tips: For me, I did this with lots of upgrades, and focused on defending the center area first, then the 2 spawn entrances with 1 berserker/shaman each. And for the bone wagon that spawns at the start, I just dropped all my spikes on it right away and let a level 2 shaman finish it off.
  Also, It seemed like the enemy spawns only increased in difficulty when you pop an eye, or when the final boss reaches a certain health threshold. So you can avoid popping them too fast in order to farm up meat for better towers first.
  Also, it helps to pay attention during your 1st clear of the map what enemies start spawning and when as you kill the boss, that way you can plan ahead better when going for the challenge.
  
• Level 4 - Win without casting the Boulder Toss spell and without building any Colossuses or Beasts (AKA Level 3 Stonemen).
  Tips: You can however still build level 2 Stonemen. Some on the S path and 2 more around the middle were enough for me. Use the other spells as needed.
  Getting all the Stoneman Tree Upgrades is also recommended.
  
• Level 5 - Get at least 14 towers under a Keeper's Range buff at once
  Tips: Reminder that Spell Cooldowns are reduced on this level, so just keep throwing them out to deal with the early waves while you place level 2 keepers around the map to buff the pre-placed towers till you complete the challenge. The level was a breeze after that
  
• Level 6 - Win without building any level 3 Berserkers or level 4 Ice Shamans.
  Tips: You're gonna need to place a good amount of level 2 hunters around. And make sure to build up enough towers on the lower path to deal with the Boss's head when it pops off. Blood Shamans work well vs it, especially if you can build 2-3 Blood shamans in a spot where you can Rage totem them all in range of the boss's head.
  And while the Horror burst spell cannot fear the boss, the ice spikes it spawns with its upgrade can still deal some extra damage.
  I recommended getting every spike trap tree upgrade as well.
  
• Level 7 - Have 3 or more Meat Golems on the map at once
  Tips: Meat Golems are the level 4 blood shaman's Ultimate ability, so you will need both of those unlocked first. Then for the mission, just remember to save enough food for three level 4 blood shamans and the ability, as they cost more then 1000 each.
  
• Level 8 - Win without building any Colossuses or casting the Horror Burst or Rage Totem spells
  Tips: You can still build level 2 Stonemen and level 3/4 Beastman. So a couple of those at the start can still help you with swarms. Then just avoid casting horror or totem by mistake and you are good!
  
• Level 9 - Win without building any level 3 or higher Shamans
  Tips: Simple enough challenge. Just build around the pre-placed rage totems and make use of the huge starting food supply you have.
  
• Level 10 - Win building only a maximum of 1 tower of each type of level 3 tower?
  Tips: I'm still not 100% sure, but I only built 1 of each level 3 tower to be safe, and I got it.
  
• Level 11 - Only build 4 or less Stoneman towers
  Tips: A pretty easy challenge honestly. I just made a kill-zone near the upper right area with 2 colossus's and a beast, and later added 1 more beast near the S spawn for the later waves.
  Just be careful of the moose type enemies that spawn from the NW spawn point on the last wave that will fly towards the SW exit if you trigger their flying mode too early.
  
• Level 12 - Win in less than 35 Minutes
  Tips: A bit tricky, you need to balance your food between Boss DPS and mob clearing.
  I ended up with 5-6 Level 3 Hunters with the anti-air ability evenly spread out to target the boss almost all the time to damage/slow it, some shamans and berserkers later on also helped a bit.
  I won after the 30 minute mark, so it will probably be a close win even on normal.
  
• Level 13 - Win without Building Level 4 Berserkers, Blood Shamans, or Beasts
  Tips: Without those level 4 towers and their nice Ultimate abilities, you'll need to use a spread of level 3 versions of those towers in key spots to make up for that, though you can still use other level 4 towers to help carry the slack if needed.
  Also, The top half tends to get less numerous but larger enemies, while the lower half gets the opposite. Plan your towers based on that, or just make sure you pay attention the first time you clear the map so you remember the waves.
  
• Level 14 - Have all of the following on the map at once. 3 or more Meat Golems, 5 or more Colossus Stone barricades, and 11 or more Beast Death Flowers.
  Tips: A challenge that is more tricky than hard. Mostly due to the somewhat finicky nature of getting 11 death flowers at once. It was annoying enough for me that I switched to easy difficulty just for this challenge.
  Step 1: Get 3 level 4 blood shamens, then buy meat golem on each of them, I used the 2 pre-placed shamans near the exit, and the 1 at the bottom middle for this.
  Step 2: Get a couple level 3 colossuses with the stone barricade skill. I put them near the exit and the middle area. A trick I used is letting a few enemies get near the exit so the colossuses back there dropped barricades there before using spells to get rid of any leaks, that let the barricades remain in a safe area to keep the count up.
  Step 3: Get 11 Death flowers, For me, the level 4 Beast that pulled it off was one I placed just above the pre-placed bottom middle shaman I used in Step 1(So it could target both the second and third enemy paths). Just make sure you have 5 or more stone barricades on the map first before you try and get the flowers up. If needed, you could try building a second level 4 beast nearby to help. Good luck!
  
• Level 15 - Win placing 6 or less Keepers.
  Tips: Not that difficult if you can beat the base map, just focus on buffing your main killzone with 3 Keepers, then spread the rest out to buff smaller secondary zones.
  Though due to having limited Keepers, you also have a limited unit cap. So to be safe, keep at least 3 towers in reserve for when the boils appear on the boss so you can be sure you'll be able to hit each one.