This guide covers the vast majority of achievements for Blackguards 2. The ones that are missing are a work in progress - if you find the solution, post it! Many of them are buggy and will require multiple attempts even if you get the requirements correct, so it's strongly recommended to keep a good number of manual saves throughout the game, especially since certain points will diverge between mutually exclusive achievements and it will save a second playthrough if you can reload at those points.

If you're planning to get most or all of the achievements, here are the recommended early choices:

1. Make a mage Cassia with Frost 4 and Heal 4 first thing
2. Easy difficulty whenever you need it for a chieve. Pride is weakness of character.
3. Do Creator quests as soon as you can get to them
4. After you get your Frost 4 and Heal 4, start on achievements in the Skilled section. The game only gets harder with time.

Good luck, and happy hunting

All of these will be unlocked by completing the game
The Gladiator
The Dwarf
The Mage
The Silent Legion
An Army
The Last Days of a Kyrios
Keeper of Melodies
Master Creator

All of these will be unlocked by conquering every point on the map & completing the game
The Impalers
Can’t Be Stopped in Melee
Deadly Rain
The Manticore
The Conqueror
The Last Days of a Kyrios

All of these are easily acquired along the way, though some may require reloading or be mutually exclusive with other achievements.

General
Amazeroth's Influence
Corrupt Zurabaran as hard as possible. Give him the melodies of the creators in the end and a chest full of gold and whatever he needs to build chimera. credit to This feels like LP already; untested.
No Holding Back
Complete all town mercenary missions. credit to Duodecimus; untested.
You Too, Scribe
Talk to merchant when first entering camp.
Kyria for a Day
Complete the game without going to extreme dark side credit to Duodecimus; untested. In the epilogue, do all the types of things that will make people happy. Cut taxes, abolish slavery, and so on. Both this and Tyrant for a Day are doable in one playthrough for a relatively middle-of-the-road character by loading back after the game ends and choosing the opposite of everything.
Tyrant for a Day
In the epilogue, do all the types of things that will make people angry. Raise taxes, abolish marriage, create more monsters, and so on. Both this and Kyria for a Day are doable in one playthrough for a relatively middle-of-the-road character by loading back after the game ends and choosing the opposite of everything.
Practice Makes Perfect
Complete all five practice missions on the camp screen. The final one is Carnage.
Rahja's Blessing
May only require seducing Niobe with dialogue choices rather than love potion or torture. After she refuses to give you the last cure ingredient, ask Zurbaran to seduce her. After that, find them in the various towns and complete her dialogue such that she gives a positive response.

• When asked what she thinks of Cassia, after her response give Serious answer and leave without asking about the cure.[/u]
  
• When she says you don't give up, give I'll leave you alone
  
• Visit her again in camp

Gold is Power!
Talk to Naurim in the camp. Choose the option to manipulate him and continually raise his anger until he says something like: "Yeah, I will walk through fire for you!" (credit to antja; untested)

Cassia Levelup Achievements
In order to complete all three of these in one run, save up 7000 points on Cassia, then quicksave and quickload to complete them all in turn. After that, the points may be spent as you wish.
The Studies
Max 6 Spells
The Hunt
All ranged Special Maneuvers – NOT Special Abilities
The Fight
Death Blow & Blade Storm

Dead or Alive Achievements
These achievements only count story deaths. A character can die on the field and still count for the achievement.
Farewell, Poultice-Head
Execute Zurbaran in the Epilogue. Can be combined with Freedom! by quicksaving before executing him.
Freedom!
Keep Zurbaran alive through the end of the game.
The Block of Stone
Execute Naurim in the Epilogue. Can be combined with Tough Dog by quicksaving before executing him.
Tough Dog
Keep Naurim alive through the end of the game.
Son of Gods
Bugged. Presumably accomplished by keeping Takate alive through the end of the game but you'll need to catch the engine on a good day to have it work.
A Life for a Life
Bugged. Presumably accomplished near the end by telling Takate to drop into the pit with Mirai but you'll need to catch the engine on a good day to have it work.

All of these will require some skill. None of them are Purple Coin Challenge level and should be completable by a typical player.

General
Broodmother
Save Spider in Prologue. Keep him in in the backfield except for fights where he needs to kill something unreachable to the others.
No Armor
Win a fight without any armor on any non-mercenary character. Easiest to do with a mage Cassia right after she & Zurbaran get Ice 4. Often takes several wins before it counts.
No WeaponsWin a fight without any weapons on any non-mercenary character. Easiest to do with a mage Cassia right after she & Zurbaran get Ice 4. May also award at random even with weapons equipped.
This is Mengbilla!
Credit to kalder0n on trueachievements.com
This is most easily completed on Hatchery Kokeni, but anywhere with a bridge and sufficient (read: respawning) enemies will work. Use Magic Wall to bottleneck enemies into position between a melee character and the edge. Next, use Liberating Blow to knock them into the abyss. Repeat 20 times.
No Blood! No Blood!
Simply finish a fight without attacking. Easiest on the fight in Naurim's treasure room very early on, but any fight with victory conditions that don't include violence will suffice in a pinch.

Master of Sand/Bones/Insects/Forest/Creatures
For these achievements, kill the Creators as early as possible. This maximizes the time you have to get controls on the various creatures. Pay the gold to Riz for Lay of the Land whenever you find him in a new town and then mouse over fights and avoid those that have monsters you need but don't have the melody for. There's not a lot leeway in getting the 20 captures, so don't ignore one because it seems too difficult.

Note: It is not necessary to get the books during the fight with the Creator bosses.
Note II: If the monster dies before the fight ends, it won't count.

All of these are completed in essentially the same fashion. Get the monster in question into the purple ring (not all maps have one - if there's no ring, there's no way to do the achievement on that map) and then have one character use the Hell Organ. If you have the right melody, it'll make the monster friendly. You will not, however, get control of it. That means it will merrily wander off and die if there are enemies still on the field, so make sure you do the capture last or are otherwise prepared to keep it alive.

Master of Sand
Sand ghosts are mostly a matter of patience. Their charge attack means they will rush to a random character every few turns, so just pile all your characters except the one at the Hell Organ into the circle and wait.
Master of the Forest
Slow-moving tree monsters covered in bees. Start by removing the bee hives at range and then pile all your characters except the one at the Hell Organ into the circle and wait.
Master of Insects
Insectoids are by far the most common, but most are on levels without circles. They are not particularly threatening, especially in small numbers, so use them to pad out your 20. They will, however, die almost instantly if they charge their erstwhile fellows, making it particularly important not to change them too early.
Master of Bones
Leapers are without a doubt the hardest of the bunch, since they will jump on groups of characters and lifedrain them for several turns, meaning that once they get to the wrong place they'll be deeply unkind. There's little to no way around this, just use the time they're busy to get everyone that isn't being turned into delicious food paste over to the center of the purple circle where the Leaper will land on them and be ready to change over.
Master of Creatures
If you're skilled & prepared enough in advance of the fight with Shinna, you can theoretically complete this one there. She drops hordes of insects on you, which should be changeable once you open both shields.

• Once someone is on the Hell Organ, you can let that shield back up to have characters free to play bait.
  
• Lightning Find You will go a long way to extending your lifespan without harming the bugs.
  
• Avoid doing damage to the bugs you want to change, since they're going to need to survive long enough for you to kill the boss.
  
• When you have as many as you need (or feel confident having) the bridges at the edges can be burned, saving your bugs from getting overwhelmed by other bugs.

These are all achievements with long, nitpicky requirements. Some may be bugged, as it's difficult to be 100% sure you've completed the requirements so people naturally cry bug early and often, particularly in a game with this many real, documented bugs.

General
What did they do to you?
A number of couriers will pop up seemingly at random in unrelated fights. Kill them before they escape to get the achievement.
Plundering
Treasure chests occur in many battlegrounds throughout the game. Find them all and pick them up before the fight ends to get the achievement. Be sure to hit V and check every square inch of every battleground, as they hide in plain sight. Note that Faramud's missions in each town include chests that are almost certainly part of this achievement.

Special
The Saint
The Monster
As they are described, it's likely that these are pure good and pure evil runs. Unfortunately, because of the sheer number of choices it's functionally impossible to know what choice leads to what outcome unless the developers come out and tell us. Many forumgoers believe this to be bugged, having done several attempts at both achievement and not gotten either, but since Blackguards 2 bugs have a nasty habit of only happening sometimes instead of just breaking things permanently there's no way of knowing whether they made a mistake or ran into a genuine bug.

In short, if you feel the need to try for these, good luck.

These are simply achievements with no found solution. If you know how to do them please put your solution in the comments along with how sure you are that it works.

A Family Dwarf
Eternal Gratitude!