A guide in my own words of how to get each achievement.
Specific campaign related achievements are in their campaign order, other achievements are in alphabetic order.
Edward’s campaign
Black Knight
You have to lose the first campaign battle. It might take some time since those goblins are quite weak compared to Edward.
Ogre’s Head
Win the Ogre Lair campaign battle, you will get Ogre’s Head trophie and the achievement for it.
Wolf’s Problem
Win the Wolf Problem campaign battle.
Dwarven army
Ressurect a dwarf buried in south west part of Silverhall campaign battle map. There are no indications of the burials, so you will have to search. The quickest way is to use invisibility potion on high priest and teleport him to the right part of the map, so you can do some corpse searching in peace.
Saviour of the Kingdom
Finish Edward’s campaign.
Krill’s campaign
Bonnies!
Summon bone dragon. The easiest way would be in Haunted Church campaign battle. The corpse needed for summoning is located in the water in north west corner of map.
Might also be possible to summon it from some other dragon corpse, I didn’t try that (let me know if true).
Wight
Lose Kingdom Patrol campaign battle.
March of the Dead
Complete Krill’s campaign.
Both campaigns
Battle lost
Lose any battle.
Death Seeker
Win a campaign on highest difficulty. I did it with Edward’s campaign, used dwarfs to build barricades to hold of undead hordes enough to sufficiently kill them. But there can be multiple other ways, of course.
Elite
Upgrade a unit to elite rank. Normally experience is gathered over the battles. If not, you can also discover Moon Temple (aztec’y looking like pyramid) with Moon Well where you can exchange valuables to unit experience. And a Transmuter’s Laboratory, where you can make your valuables more valuable.
Halloween!
You get a pumpkin item in 30th October (you can also skip forward to 30th October by using your handy OS time travel feature – system clock).
McZombie Always fresh
Use a ghoul to eat a zombie. Easier in Krill’s campaign, since you will have easier access to undead units.
Mimic
Find a Mimic. To do that destroy the chests in random battles until one will turn into Mimic, when destroyed.
Modder!
Upload game’s mod on Steam. It can be a placeholder/empty mod. Launch game from Steam library screen, choose “Workshop Manager”. “Create new workshop item”. Continue with Workshop manager, choose “Open folder”. Open config.xml file, change “CHANGE_ME” values to whatever else, save. Open preview.jpg, resize to 100×100. In Workshop Manager press “Upload to Steam”. It’s done.
Puppet Master
Take control over enemy unit. Can be done in multiple ways. Easier in Krill’s campaign, since you start with “Command Undead” ability. Bribing will also do in both campaigns.
Purple Heart Your Hero has been hurt!
Get Purple Heart trait for any of your heroes. You can get it when getting injured in battle.
Scholar
Read all of the game’s guide. You can open it in battle, by pressing escape, there you have the option to open game’s guide, just click through all the pages and you are done.
Snowman
Play the game in December, pick a random fight in snowy terrain, deal damage to snowman. You can just use your OS ability to travel in time by changing system clock.
Time Traveller
Use the spell with the same icon on any of your units. Archmages has this spell by default. They are easier to get on Edward’s campaign, they will be available later in campaign in your unit roster and Marcos will turn into one.
Treasurer
Save up 10 thousand gold. Easiest way is just to unlock as many gold earning locations that you can, it will increase your income significantly.
Undead Minion
Raise an undead minion. Easier in Krill’s campaign, since he’s a necromancer himself and can raise an undead minion since the beginning of his campaign.
DLC achievements
Captain Red Beard!
Start Pirates! DLC campaign. To do it there’s a tavern location on the map, start talking with the drunken sailor there.
Dragon Seal!
Get both parts of the Dragon Seal. To do it you have to play the Quest for Immortality DLC campaign until you beat the boss crab. You get very good weapon (double damage and two attacks) and the last part of the seal.
