Please let me know if you see any errors so I can correct them.

I won't be hiding any spoilers in this guide as it would make it harder for everyone to read.
If you don't want to be spoiled (bosses, tactics, secrets...) stop reading now.

Else, enjoy ! Hope it helps !

04/07/2015 : Updated with infos from July 2nd patch.
16/07/2015 : The guide is now complete with informations on every achievements.

Reach Level 10

Self-explanatory. Will come through natural progression.

Reach Level 30


Self-explanatory. Will come through natural progression.

Reach Level 50


Self-explanatory. Will come through natural progression.

Destroy 100 pumpkins


This should come through natural progression. However, if you need to farm it, you have a lot of pumpkins on the left side of The Onion Quarter.

Complete 5 normal challenges on a single map


Self-explanatory. See Tenacious V for more infos.

Complete all normal challenges in 20 different locations


Self-explanatory. See Tenacious V for more infos.

Complete all normal challenges


Self-explanatory. I don't think any of the challenges are too hard as long as you're trying enough. Though if you are having trouble with one, don't hesitate to tell me and I'll try to fill this section with tips.

Complete an elite challenge


Self-explanatory. See Been There, Done That for more infos.

Complete all elite challenges


Self-explanatory. Same as Tenacious V, if you feel like you need help for some of them, ask away.

Find all secrets in the Hunting Grounds


Self-explanatory. See Know-it-All for more infos.

Find all secrets in 20 different locations


Self-explanatory. See Know-it-All for more infos.

Find all secrets


Self-explanatory. If you need help to find secrets, check out this guide.

Equip a Legendary weapon


Self-explanatory. You will get a Legendary weapon eventually. If you don't want to spend too much time killing monsters to drop one, you can buy the Pumpkin Hammer from GebaN (see Steamed Up for more infos).

Transmute a perfect weapon


You can transmute weapons and other things in the Hub. A perfect weapon means a weapon with all its stats maxed. Just follow the instructions and keep upgrading it until you reach the maximum everywhere.

Shoot 250 chickens


To do this achievement, you'll have to obtain the Legendary weapon Chicken-Cannon and shoot with it.

Dance with the skeletons


All you have to do is get hit by a Dancing Skeleton. There is a chance that you will start dancing after getting hit and you will unlock the achievement. You can find Dancing Skeletons in Widow's Mine, Weeping Halls, The High Quarter, Cemetery Grounds, Forlorn Catacomb, Defiled Chapel, and The Merchant Quarter.

Set foot in every area in the game


Self-explanatory. You should naturally unlock this achievement.

Have 6 outfits


Outfits are now drops. Keep playing and you'll get them. If you had them already because you were playing before the patch, the achievement should unlock automatically.

Collect all the Fool Destiny Cards


There are 10 different Fool cards. This will come with some grinding. Keep playing and hope to drop them. Check Patriarch Casimir's shop often as you may find one to buy there.

Collect 25 different Destiny Cards


You have to have 25 different Destiny Cards in your inventory (not your Private Stash) and it will unlock.

Inflict 2500 damage with a single hit


Self-explanatory. Will come through natural progression. If you still don't have it, try using Fire Lake (Hand Mortar skill) in an area with a lot of monsters and kite them inside.

Have 6 equipped Divine cards


Self-explanatory.

Acquire 150 000 pieces of Gold


150 000 is a relatively small number. This will come through natural progression.

Trade with GebaN


GebaN is a boss that you can find on the top-left corner of Cemetery Grounds. Kill every monsters around the checkpoint and then go up where the gargoyles are. Keep killing everything. Eventually, GebaN will spawn. When you kill him, he'll turn himself into a merchant. He sells a small number of items including a unique Legendary weapon : Pumpkin Hammer. Just buy or sell something and the achievement will unlock.
You can also find him around the northeast corner of The Merchant Quarter. The tactic is the same. Kill everything including him to be able to trade with him. He sells a different Legendary weapon in The Merchant Quarter, a hand mortar called Chimera.

Step on 10 Mites within 5 seconds


Mites are very small green spiders. They are harmless and you can kill them with weapons or by stepping on them. The easiest way to do this is during the fight with Mordra the Broodmother. She will spawn a lot of them, just step on them as quickly as you can.

Reach 250 Armor


This achievement is doable by equipping multiple copies of "The Knight" Destiny Card (gives flat armor) with "The Smith" (gives armor while wearing a hammer). Not sure if "The Acrobat" works.

Make 2500 kills with 5 active Hexes


See Serious Vran for more infos.

Make 15000 kills with 5 active Hexes


If you like to play around with higher difficulty this achievement should come naturally while playing. However, if you want to farm it, you can go in areas with a lot of "easy" monsters to defeat, such as Befouled Tomb and restart the area after killing everything.

Kill 25 enemies with a single attack


Try to pack some monsters and use your AoEs effectively. Some easy skills to use would be Point-Blank Shot on the Shotgun, Whirlwind on the Scythe or Fire Lake on the Hand Mortar.

Kill 50 enemies with a single demon power


An easy way to do this is to go in the Mausoleum of Bones. On the top right part of the map, after you defeat a bunch of Undead Pyromancers, you'll have a LOT of skeletons spawning. Simply aggroing them and using Purging Flame should do the trick.

Kill 500 enemies without using consumables


This achievement is cumulative. You do not have to kill 500 enemies in the same map. As long as you don't use any consumables it will keep counting. The easiest way to do this is with ranged weapons that have Health on Hit modifier. Go in areas with a lot of easy monsters like Befouled Tomb.

Defeat 20 Champions or Tyrants


Self-explanatory. Will come through natural progression.

Have 100 monsters being killed by their peers


This will come through natural progression. The most "reliable" way to do this is to play around with monsters while a Bombardier Spider is shooting at you.

Slay the Chromatic Essence


You can find this boss in The Devil's Throat. It's an Essence of Ice. When you kill it, it will divide itself in 3 Essences of Fire that will divide themselves in 3 Essences of Lightning that will divide themselves in 3 Essences of Rock. Don't kill them too fast else you'll have a lot of them to deal with at the same time.

Slay the Screeching Calamity


This achievement cannot be missed. You can find this boss in the Defiled Chapel. There is no special tactic except killing everything. Be careful not to stay too much in AoE bleeding / frailty.

Have 25 consecutive split-timed attacks


This achievement is not as hard as it sounds. You just have to time your attacks correctly.

xylto の投稿を引用：

Holding down the fire button with a shotgun will never make a split-timed attack. You have to attack exactly the moment when the cooldown ends, if you press attack during the cooldown it doesn't work. It also seems that getting hit by any attack will end the chain.

Kirkkaf の投稿を引用：

Steady Hand Vic can be unlocked upstairs in the castle on the training dummies.

Make 5 Slash attacks in 3 seconds


The Sword's Slash attack automatically recharges when you crit an enemy. The easiest way to do this is by equipping the Legendary consumable "Lucky Rabbit's Foot" which gives 100% crit chance for 5 seconds.
If you don't have this consumable, try maximizing your crit chance with multiple copies of "Strength" Destiny Card in combination with the Legendary weapon Lady Luck. This weapon already has 40% crit chance on her own. While the Destiny Cards are not necessary, they will make the unlocking of this achievement easier. After that, it's only a question of luck. Find a bulky monster (or activate Hex of Tenacity) and spam Slash. Retry until the achievement pops.

JinHit の投稿を引用：

Same as for the Shotgun with dummies. On 3rd hit you apply debuff, so you simply do it on 4 dummies, top or bottom row. When all 4 got debuff, stand in the middle and use "Slash" on 4 of them and with 5th Slash you can hit a thin air, doesn't matter. Done =)

Make 10 Aimed Shots in 3 seconds


The Shotgun's Aimed Shot attack automatically recharges when an enemy is killed with the Shotgun. The easiest way to do this is by spamming Aimed Shot at the Mites (as you will one shot them) during the fight with Mordra the Broodmother.

Charge 4 times within 8 seconds

To be able to do this, you need the Legendary weapon Power Foil.
The special modifier of this weapon is that you can use Charge 2 times without cooldown. Equip Ambrosia potions. Use Charge 2 times, use the potion, use Charge 2 times again and the achievement should unlock.

Hit enemies with 10 consecutive Mortar attacks


Use Fire Lake on a big pack of monsters and this should unlock.

Defeat Lord Gabriel


This achievement cannot be missed. You can find him in Sanctum Sanctorum. The fight is slightly tricky, just learn the patterns.

Defeat Lord Gabriel on Hard Mode


Self-explanatory. Hard Mode is accessible when you create a character. You can not switch Hard Mode on and off like hexes.

Defeat Lord Gabriel with a character who never died


Self-explanatory.

Defeat Rachnoss the Host


This achievement cannot be missed. You can find this boss in The Tangle. To kill it, you need to destroy the columns so it falls on the ground. Let me know if you need a walkthrough of the fight.

Defeat Mordra the Broodmother


This achievement cannot be missed. You can find this boss in the Den of the Broodmother. There is no special tactic except killing everything.

Defeat Vail the Immortal


This achievement cannot be missed. You can find this boss in the Mausoleum of Bones. Let me know if you need a walkthrough of the fight.

Defeat Adrian The Fallen


This achievement cannot be missed. It will come during the story. Let me know if you need a walkthrough of the fight.

Defeat Wa-Lee the Solicitor


This achievement cannot be missed. You can find this boss in the Demon World. Let me know if you need a walkthrough of the fight.

Defeat Apocalypse


This achievement cannot be missed. It will come during the story. Let me know if you need a walkthrough of the fight.

Defeat Adrian The Fallen with a party of 4 in multiplayer


Self-explanatory.