The game features two distinct campaigns - in one you play a Human Knight and in the other a Necromancer - each with its own set of achievements. Playing through the two campaigns would get you essentially every achievement apart from Treasurer and Death Seeker unless you are some sort of a superhuman who would get both in the first playthrough. This is not impossible but I would say that it is extremely extremely hard. Therefore I would suggest to play through both campaigns on normal difficulty - which is a good compromise regarding challenge and gameplay - after which you could do a quick partial runthrough of the Sir Edward campaign to get Treasurer and finally a separate runthrough of either campaign on Death Seeker difficulty to get the final achievement. This roadmap will structure the achievements in the above order but of course it is possible to deviate from it. If you play the Sir Edward campaign on easy then you could easily bag Trasurer in your first campaign as well leaving you with the Undead campaign and Death Seeker. Or you could just start on Death Seeker difficulty and collect everything on your first playthrough. As the achievement description says ... I wish you a good luck for that.

A final comment on the achievements. Some of these are supposed to be secret ones so if you want to inch through your way the battle maps finding them then please do not use this guide. In my opinion these are close to impossible to find unless you know what you are looking for and at least the campaign and mission where you are supposed to find them as the name of the achievement does not give enough hint.

SCHOLAR
As you start the first campaign and get in your first battle go to the Menu screen and then press Help. You need to read through all 18 pages of help / tutorial to get this achievement.

PURPLE HEART
Very simply move your hero in the very first battle of the Sir Edward campaign close to an enemy and let them wound you. Getting you killed by the attacking goblins would catapult you on the Black Knight campaign while killing the goblins kickstart the normal Paladin path.

BLACK KNIGHT
As described above simply let you get killed in the very first battle of the Sir Edward campaign. The story will resurrect you as a Black Knight and you get the achievement. Missions and battles are essentially the same thereafter as in the normal campaign only difference is that the Black Knight character has a slightly different skillset (in my opinion much stronger in melee especially if the cursed sword is also equipped later)

OGRE'S HEAD
Finish Battle 2 of the Tale of Sir Edward campaign after you did the first battle anew as a black knight. You need to cut a certain bodypart of a certain game protagonist to achieve this. Achievement is unlocked as you get the resulting item after the end of the mission.

WOLF'S PROBLEM
You need to finish Battle 3 of the Tale of Sir Edward campaign. Certain furry enemies need to be killed. Achievement unlocks as you receive the resulting item after the mission.

BATTLE LOST
This achievement is first available after Battle 4 - Road Bandits. You need to lose the battle for the achievement to pop. Losing the battle of course means a reload because eventually you need to win it to proceed with the story.

ELITE
Units normally have three upgrade levels with each level giving certain bonuses such as attack, defence, morale, HP or MP in case of spellcasters. Elite is the third upgrade level symbolized by a small golden cross on the unit icon that comes after Regular and Veteran. Reach elite status with any of your units to get this achievement.

TIME TRAVELLER
Using the advanced mage skill 'Timeshift' will result in this achievement. For this you need to acquire a mage, develop the mage into an archmage, equip the archmage with the Timeshift spell and use it once in a battle. Normally this will not happen before one of the very last battles (most probably the first battle of the last mission) of the Sir Edward campaign.

DWARVEN ARMY
One of the more difficult secret achievements that personally I think is not possible to do without a guide or at least some pointers. You need to find some frozen dwarven warriors and resurrect them to life using the resurrect skills of your high priests. Aforementioned dwarfs are found in Sir Edward campaign battle 13 (Silverhall battle in the frozen wastelands). You can resurrect the dwarfs either during the actual battle (will be more difficult as your priests are rather vulnerable) or after the battle. In this latter case make sure to leave a high priest with an unspent resurrect skill. The other important bit is that these secrets cannot be found the normal way by your knights horsing around the battlefield (this actually gave me a lot of frustration). You need to head South just under the hilly area directly from your starting location using your high priest. When you are roughly in the right area you need to trigger the resurrect skill to see the skills effect circle. In case you are in the right area the bodies of the frozen dwarf warriors will show as white circles on white background (just to make it any easier). Click one of those circles and resurrect at least one dwarf which will join you for the rest of the game. You only need to resurrect one dwarf though for the achievement to trigger.

SAVIOR OF THE KINGDOM
Winning the 15 battles of the Sir Edward campaign on any difficulty will net you this achievement.

UNDEAD MINION
This is most probably the first achievement you can get in the Necromancer campaign. Your original hero Krill has the Raise Dead skill from the beginning. You need to use this skill on one of your own killed undead or on enemy corpse (to raise a skeleton). By the 2nd or 3rd battle you should surely have some Zombie troops and when one of those falls in battle you should simply get your Necromancer to use the Raise Dead skill on them.

PUPPET MASTER
Battle 4 of the Necromancer campaign is going to be the first battle where you fight undead enemies. You need to use Krill's Control Undead skill to take control of one of the enemy undead creatures and the achievement pops.
Note: this achievement could be also gained by exercising any of control spells (in any campaign), however it depends on randomly dropped items for your hero.

BONNIES!
This is supposed to be a secret that nobody should find on his own so read further at your own peril - I warned you. After winning Battle 4 Haunted Church (the one with the banshees) you need to look with your Necromancer in the mud pit (with the bubbles) in the top left side of the map. If you use the Raise Dead skill of your necromancer surprisingly you will see a very large circle which in fact contains your very own pet - a Bone Dragon. Smile because you can keep this creature in your army and having a giant tank with an HP of 14 this early in the campaign should really brighten up your day.

WIGHT
Similar to Black Knight in the Sir Edward campaign you can change identity and slightly the story flow if you lose Battle 5 Kingdom Patrol and turn into a wight. Just start Mission 5 and do nothing. You will die and come back to life - or rather death - as a wight - achievement pops. I suggest reloading after this and winning the Battle to continue as Krill.

McZOMBIE
Ghouls are first available in Battle 7 Greenskin Dispute of the Necromancer campaign. You can either grab a ghoul to be part of your team or evolve one of your zombies into a ghoul. Ghouls have a skill called Consume Corpse which is essentially a healing spell for ghouls that adds 2 HP back. Use this skill in Battle 7 on any dead enemy corpse and the achievement is yours.

MARCH OF THE DEAD
After you have won the 14th last battle of the Undead campaign this achievement will pop.

This achievement is only available playing the Sir Edward campaign as there is no money involved in the Undead campaign. Once your treasury shows over 10000 gold you will get this achievement. This is in fact harder than it sounds as it costs money to replace your fallen troops. Essentially the following three things could facilitate collection 10000 unspent gold:

1. Playing the campaign on easy difficulty where the cost of troops is cheaper compared to the other difficulties
   
2. Investing in the Merchant skill for Sir Edward which means a 20% discount on all troop costs
   
3. Getting all the visible and hidden treasure on each of the maps could get you an extra roughly 3000 gold thrughout the campaign

Using the above aids and some sensible management of your troops you would be able hoard the required 10000 gold by Battle 10 or even earlier.

Your last playthrough and most probably your last achievement is going to be the on Deathseeker difficulty. The following comments are directly from Les (developer of the game) and I cannot add anymore as you would need all your knowledge of the game, cunning and luck to beat this one.

'It is designed to be pain in ♥♥♥... In-battle save is disabled (no more save-scumming!) and enemy has stats like on hard.

Your units will usually have lower stats, therefore Critical Hit, Shield Breaker, Vorpal Blades, Lifedrain (always 90% hit) skills are now your best bet.

Second, your units get permanent wounds lowering their stats, thus you want to avoid damage as much as possible. Playing humans focus on ranged units and mages, playing Undead just keep replacing units. Additionally, heroes don't get permanent wounds, thus you can tank with Sir Edward.

Third, you will need every advantage - search every map for artifacts, steal items (humans - almost any soldier can steal, undead - banshee and shade). Use fire-attacks on mummies, melee mages, etc.

I will suggest to play humans and tanking with Sir Edward. Just make sure you find all hidden chests to equip Sir Edward.'