Guide was to v1.0, some things are probably out of date.

• 1 - Always easiest difficulty
  
• 2 - Use pyromancer or your favourite red/chaos primary summoner for aoe dmg spells, to get through early game more quickly.
  
• 3 - Edit starting cards to 3 of these artifacts and/or spells, in descending order of strength/importance. If you haven't found/unlocked 3 of these yet, I would re-start as soon as you find 3. Don’t forget to equip the artifacts!.
  
• 4 - Do easiest biomes in this order - Dungeon – Forest – Desert. Dungeon seems to have more opportunities to find guardians, and more free equipment. Forest gives you loyal tree and goblin party, which is nice.

• 1 - You can retreat and re-enter 99% of fights:
  
  • Early game, retreat from difficult fights until you have good enough spells and/or guardians.
    
  • 50% of fight difficulty can be reduced by changing setup:
    
    • Enemy positions switch every time. You want the elite/boss "open" (fight ends as soon as you kill it), and/or minions all clumped up for AOE spell/attack. Using different entrances can make this easier: wide entrances for bosses, to isolate them; short entrances for minions to clump, or if you want to start further away from enemies.
      
    • When you enter, you can retreat risk-free. After retreating, do something else like another fight, rest, etc, and when you come back you get a different hand of cards.
      
    • After a won ambush roll, you can retreat and re-enter for a “normal” setup where everyone isn’t on top of you.
      
    • 1b (dodgy) - If you lose an ambush roll, you can close the game before clicking the accept button, and try again. This is an exploit, and you get a progressive score penalty, but we don’t care about score here.
      
      • This exploit works on other rolls like rescuing hellhound puppy, drinking potion, etc.
• 2a - You can un-do every action unless there is a surprise. (only 30% of players do this).
  
• 2b - You can discard 2 (1 with artifact) cards for mana (only 15% do this). I do both in 90% of the fights.
  
• 3 - Prioritize getting 2 guardians asap, if possible a combo that helps 1-shot elites/bosses.
  
  • My favourite is by far griffin (yellow, vigil, fly, 6 moves) and rock giant (neutral, knockback, vulnerable 2). You can also give a bow weapon to rock giant if you’re finding him hard to use, although you lose the knockback. Lunge weapon also good, cheap, and makes it easier to stack multiple knockbacks. Explosive fireball into (buffed) griffin 1-shots most elites.
• 4 - To avoid getting killed in retaliation: overkill by at least 1 damage; attack from behind; or send an expendable unit before attacking with your important/fragile ones. Also be careful with ranged vs. ranged attacks, especially if using pyromancer or staff aoe-attack, as all ranged units that survive attack back.

Lil' Hellraiser - Rescue a Hellhound puppy (6%) - the puppy is located inside the boss dungeon (not the overworld where you start), usually in the bottom-mid area. I think it only spawns when demon lord is the boss, so you’re risking wasting runs if you go harvest last and get the wrong boss. It’s also hard, so wouldn’t recommend starting harvest, even if you’re targeting this.

Arena Champion - Win your freedom from the Sandpit Arena (30%) - You’ll get captured and need to win a series of fights.

Party Animal - Become the life of a goblin party (43%) – world encounter goblin party.
Spoil the Surprise - Spoil a goblin birthday (33%) – world encounter goblin party.
Goblin Birthday Wish - Complete an adventure with birthday boy goblin alive (1.3%) – do forest as last biome and wait until you killed final boss before getting the goblin party.
Test of Loyalty - Have a loyal unit survive 5 victorious battles (44%) – look for surrounded treant before doing any fights. Careful with fire-friendly-fire.

Revenant Regicide - Defeat the Liche King in adventure mode (26%) – one of the Dungeon bosses. If you start reading lore about a vampire, the liche king will not spawn.
Talent Acquisition Specialist - Recruit 30 different guardians (5%) – remember you can only get guardians from the color/type of your summoner. Go to 'Progression' – 'cards' - 'filter guardians' to see which ones you are missing.

To get these quicker, choose a summoner which has the first color/type of that section. Most cards you get are from the first color of your summoner, especially early game. Certain colors have more cards with some effects.
To find cards of certain type/effect, go to 'Progression' – 'cards' – 'sort' – 'search' for vigil/surprise/whatever you need

Chaotic - Cast 150 Chaos cards (18%)
Demon Conjurer - Summon 20 demons (22%) – also void/black
Demon Lord - Summon 50 demons (5%) – also void/black
Hoarder - Summon 30 orcs or goblins (11%) – also freewill/neutral (goblins)
Spell Damager - Deal 200 spell damage (44%)
Spell Destroyer - Deal 500 spell damage (24%)
The Final Attack - Use Downfall 10 times (49%)

Celestial Army - Summon 30 Celestial units (15%)
Time Traveler - Send a unit to the future 5 times (7%)
Combo King - Trigger 50 combos (17%) - Celestial for vigil (other colors also ok); Chaos/red or equipment for knockback.
Triple Combo - Perform a triple or higher combo (22%) – as combo king. Need multiple vigils or knockbacks.
Surprise - Surprise the enemy 3 times (44%)
Ultimate Surprise - Surprise the enemy 30 times (7%)
Strongest Shield - Block 50 attacks (25%)
Supporter - Support 20 allies (26%)

Sacrificer - Destroy 5 allies (34%)
Necessary Sacrifice - Destroy 20 allies (12%)
Undead Army - Summon 100 undead (17%)
Void Master - Cast 150 Void cards (12%)
Assassin - Backstab 150 enemies (36%)
Lifestealer - Lifesteal 50 health (29%) – 3 void, 2 chaos cards
Warcryer - Activate warcry 30 times (15%) – many freewill/neutral cards, a couple from each color
Debuffer - Debuff 250 enemies (29%)

Arcane Apprentice - Cast 30 Arcane cards (26%)
Cold Hearted - Freeze 30 enemies (33%)
Elemental Apprentice - Summon 20 elementals (42%)
Elemental Master - Summon 50 elementals (22%) – chaos/red and nature/green also ok
Ice Fall - Deal 50 ice damage (32%)
Mind Bender - Mind control 10 enemies (21%)
Mind Destroyer - Mind control 50 enemies (7%)
Psionist - Deal 50 psychic damage (26%)
Seer of the Unknown - Draw 30 cards with magic or abilities (9%) – also nature/green

Animal Lover - Summon 150 animals (17%)
Creepy Crawlers - Summon 10 insectoids (41%)
Frankenstein's Apprentice - Mutate or transform 10 allies (44%)
Frankenstein's Successor - Mutate or transform 50 units (12%)
Master of Nature - Cast 150 Nature cards (14%)
Nature Strikes Back - Summon 10 plants (24%)
Poisoner - Deal 100 poison damage (17%) – also with void/black card 'poison cloud'
Squirrel Hurler - Hurl 100 squirrels (41%) – druid summoner
Super Healer - Heal allies 500 life (21%)
Stunner - Daze or immobilize 50 enemies (22%) – most colors have a bit, nature/green has great immobilize.
Super Buffed - Buff 150 allies (28%) – most colors have a bit

n/a

Choose Wisely - Take a sip from the Holy Grail (26%) – world event - alchemy table (one of them). procs on certain type/colors, not sure which. Definitely NOT with pyromancer (chaos/celestial). might proc only in dungeon or harvest biomes, but not sure.

After you have a good understanding of the game, and a powerful start with 3 artifacts, watch out for opportunities to do these hard ones:

Challenger - Defeat a boss without using soul gems (1.4%)
I recommend the usual 'dungeon –> forest –> desert', and go for it when you have an unusual strong roster and/or you find potion lady shop, and spam mana/+2+0 potions to help. satchel units (elementals etc) also helpful. Careful with money chests and such things that auto-loot gems/everything around. Alternatively, start with the "summon goblin party" artifact and summon them before the first boss.
Lone Wolf - Complete an adventure without recruiting any guardians (0.5%)
As above, & abuse loyal (hellhound puppy from artifact "get any unlocked card"; treant and goblin party from forest biome) and items (you can re-equip).
Early Retirement - Buy out all the cards in a shop (3%)
Take artifact that gives 60 gold per fight, stop spending gold when you have 2 guardians and good (/cheap) equipment. Look for a shop with fewer/cheap cards.

Walk 500 Miles...OK 100 - Walk 132000 tiles (0.3%) – find 3-4 connected map sections that you can walk without being blocked, then watch LotR triology extended cut on 2nd monitor while pressing left/right.
I finished the game and all other achievements in ~60h (some afk) while having 37k/132k steps..

A second warning to completionists/100%'s:

Only 0.1% get the last survival achievement. The developer has, in the past, and might again: add more/harder waves to survival, add new achievements, nerf broken strategies, patch bugs/abuses, etc.
This is a great thing for the game and everyone, but is a bit annoying for completionists. On the upside, there might also be new things to help, like the newly re-vamped equipment system and new artifacts.
This is all to say that I recommend hard cheesing survival mode when going for the achievements. For other/newer ideas, try the discord. This is how I did it:

Survivalist - Clear 8 waves in survival mode (1.2%)
Survivalist Godan - Clear 16 waves in survival mode (0.4%)
Survivalist Judan - Clear 20 waves in survival mode (0.3%)
Survivalist Shodan - Clear 12 waves in survival mode (0.4%)
Survivalist Wave 24 - Clear 24 waves in survival mode (0.3%)
Survivalist Wave 30 - Clear 30 waves in survival mode (0.1%)
Survivalist Wave 35 - Clear 35 waves in survival mode (0.1%)
Survivalist Wave 40 - Clear 40 waves in survival mode (0.1%)

As described above, this is not a "effective strategy" section, nor something I would recommend if you're playing for fun. It's a single cheese approach that worked in a certain patch to get a very hard achievement.

Necessary satchels: Play from discard x1 (scribe does not work), 2 mana x5, recover cards from discard x5. "If no healers, each party member heals 5" satchel also useful.
Necessary cards: 'Blinded by the light', 'Freeze time', 'Blue Dragon'x2, 'Occuloid'x2.
Helpful cards: units with knockback; big hitters, preferably with vigil. 'red starling' and 'centurion' for +1+1.

Early, you want to spam the board with all the units. To (re)build the board, stall by keeping the last enemy unit busy with auramancer forcefield, or cosme iron soldier, or frost dragon. I did it wave 1 or 2, and never had to rebuild. Once you have the board filled, you kill everything turn 1 without letting enemies attack. Occasionally stall to catch satchels.

The biggest difficulty spike comes when enemies start invisible. Before this happens, you need to find a satchel with artifact "play 1 spell from discard per battle" for the anti-stealth 'blinded by the light', which is cast as the first move of every wave.
Bosses always die turn 1 surrounded by blue dragon (30 dmg) and golden dragon (~10 dmg with vigil and knock 1). They always spawn in the "middle" of the wave.
Another big difficulty spike is the flesh golem that spawns in most later boss waves, and prevents you from 1-shotting the boss. To kill it, 1 Occuloid diagonal attack was usually enough to trigger enough knockback combos into the dragons. bandit archers if needed more knockback.
Non-boss waves are actually harder in this strategy, but 'freeze time' (to play it turn 1, you need 2 mana pot + green satchel to recover cards from discard) after you used everyone buys you a free 2nd turn. I only needed to do it in 3-4 waves.

This could be optimized further and was done before the new item re-vamp, so I’m sure one could theory-craft new ideas. Rock giant (vulnerable 2) with a bow weapon seems an obvious one to kill bosses.

You should get these just by playing, or they are self-explanatory.
Add in comments if you’re having trouble and need more explanation for any of these.

Advanced Adventurer - Complete an adventure on Advanced difficulty (11%)
Boss Destroyer - Defeat 3 bosses (42%)
Boss Executioner - Defeat 10 bosses (13%)
Boss Exterminator - Defeat 25 bosses (3%)
Camper - Consume 200 marshmallows (9%)
Destroyer - Defeat 1000 units (16%)
Discarder - Discard 10 cards (14%)
Dragons Love Pitchforks - Equip a dragon with a pitchfork (2%)
Hardcore Hero - Complete an adventure on Hardcore difficulty (2%)
Harder Knocks - Knockback 150 enemies (39%)
Knighted Novice - Complete an adventure on Novice difficulty (25%)
Pencils Have Erasers - Use undo 30 times (28%)
Warlord - Win 200 battles (16%)


Battle Hardened - Win 10 battles
Blacksmith - Equip 3 allies
Boss Defeator - Defeat a boss
Buffed - Buff 30 allies
Creature Trainer - Capture and train a creature - World event
Customizer - Customize your deck
Explorer Extraordinaire - Explore an entire world
Hard Knocks - Knockback 5 enemies
Hope Unfolds - Help a fallen angel - World event
Inflicter - Inflict 40 enemies
Journey Begins - Complete the tutorial
Life of a Salesman - Rescued the Traveling Salesman - World event
Lucky Roller - Pickup 20 dice
Scholar - Complete a lore collection
Slayer - Defeat 100 units
Strategist - Use a satchel card
Worthy Kiss - Win a fairy's affection - World event

• 1 - There are a lot of achievements. Finding a playstyle that is fun for you is better than being very efficient.
  
• 2 - Playing “Fast adventure” mode with each available team unlocks a lot of cards, even if you quit right away.
  
• 3 - For basic info, check the developer guide. For more help, check the official discord.

Comment if anything is unclear, anything else I should include, or any other feedback.
Have fun!