Since description of some achievements are so "mystic" i tried to guess what devs had on mind when inventing them - thus it may be a bit unreliable to get them.
There are no achievements for completing game on specified difficulty, so i would recommend sticking to "trivial" as battles are easier there.
No DLC's are needed to achieve platinum(100%).

If someone know what to do to unlock conqueror and diplomat please let us know in comments.
I started thread on devs forum about 2 broken achievements, you can check out it here: NEOCOREGAMES[neocoregames.com]- let our voices be heard!

Only achievements for your army is for archers. So train them a lot. Kills are added to progress bar only through manual battles.

Kill 300 enemies with your archers in one battle.
Kill 500 enemies with your archers in one battle.
Kill 1000 enemies with your archers in one battle.

Yor archers has to shoot quite much in single battle (250 000+ arrows).

Killing and casting achievements are synchronised, but make sure you have abilities for all listed elements. Numbers of enemies killed are synchronised just after battle - if you kill 1000 enemies with spells in one battle, you will get achieve just after battle. Spells from captured towers and Holy Grail shards propably counts to this as well.

Kill 300 enemies with any kind of magic in one battle.
Kill 500 enemies with any kind of magic in one battle.
Kill 1000 enemies with any kind of magic in one battle.


*NOTE*You just have to cast spell, not kill/ress with it. Spells canceled by enemy magic shield does not count, invest in piercing/weakening power. Also only manual battles counts for achievements.

Cast lightning magic spells 150 times. e.g. Chain Lightning
Cast ice magic spells 100 times. e.g. Winds of North
Cast fire magic spells 100 times. e.g. Firewall
Cast shadow magic spells 50 times. e.g. Shadow Curse
Cast poison magic spells 50 times. e.g Poison Curse.
Cast ressurection 25 times. Spell "Ressurection" is unlockable through morality tree. Shard of Damnation (Morgana shard) and captured tower with spell ressurection also progress this achieve. I recommend to get morality ress as you won't be chained to certain battlefields. Also you don't have to actually ressurect anybody with it - just cast it on anybody, even with full HP.

*NOTE* You can farm spellcasting in any battle. Just make sure your hero has at least a bit of mana regeneration and spells that you need. Slaughter almost all enemies except 1 archer platoon. Let live only few archers in it (like 2 or 5 - the less the better), and make your army "Hold Ground" and "Hold Fire" commands. You can now cast spells almost infinitely. Take notice that long range units don't have ammo, so they can shoot for eternity - watch out for that, as these few archers can kill y'all after some time. Healing spells and ressurection recommended. Also you can farm spells that are not unit chained (most curses) as you sooner or later will be forced to kill all enemies with it. From my experience you can farm these with no problem:
FIRE - fire wall; fire ball
SHADOW - shadow curse;
LIGHTNING - chain lightning; thunderbolt
FROST - winds of north
NECROMANCER - ressurection

Poison achievement are only one that cannot be farmed with that strategy.

You can save scum artifact creating. Gather at least 3 artifacts and save game near Artifacts Smithy(location near Elstow). Create artifacts from them. Load previous save. Create artifacts 100 times.
If you have more artifacts, it would take less loads to acomplish that achievement.
2*x-1 is formula to create x artifacts.You create one and next merge it with two others. In this process you would need 199 artifacts to do 2 achievements without save scamming if i'm not mistaken - quite a lot...

Forge 50 artifacts.
Forge 100 artifacts.

Gather 50,000 gold through 1 campaign.
Gather 75,000 gold through 1 campaign.
Gather 100,000 gold through 1 campaign.

Read cronicles for about 1 hour. I guess...Nobody will count how long you actually read, so you can open chronicle and go AFK for some time (as i'm not sure if it is really 1h (using Cheat Engine for speeding up the game could make it faster, as game will count x-times more than actual time...))

Win 10 battles fast. Just Ctrl+A and do "rush B"! This means attack move with all units.

(feedback from our reader) Win 10 battles while holding every victory location.

You need to make 50 or more agreements during ONE playthrough. New campaign will reset progress. There is no visible progress bar in steam for this one.

Complete base game. Any diffculty.

With a bit of research, I managed to find tool(software to be precise) that can be used to unlock certain achievements. It is called SteamAchievementManager (SAM), and it can make locked achievements (buggeg or too hard for unskilled players to earn) achievable.
Search for it for yourself, you should be able to download it from github. I won't tell you how to use it, as i consider it cheating and unfair. You are using it on your own responsibility These tools are not developed by Valve, and by that you can be banned by using such tools as I was.
Whatever damage it will cause to your account/game files - you will be responsible for that and no one else.
Good luck.

Words on intellectual property:
If you want to copy this guide somewhere else for free just for sharing purpose - just add link to my steam account at the end of it with postscript, that I was a creator.
There is no permission from me of any kind to share this anywhere for profit.
If sharing it, you'll agree to not change anything of this guide - just copy it as it is.
For any other purpouse, ask me for permission.

Link to my Steam profile: http://psteamcommunity.yuanyoumao.com/profiles/76561198018122414/

You can support me at: https://ko-fi.com/sh33p_2587

Thank you for reading, and have a nice day.
-Sincerely,
Sh33p.

Russian Forum[kingarthur.hop.ru] that explained a few achievements.

Exophase[www.exophase.com] for achievement pictures.

Thanks to Masta Bandee for linking this:
Paradox forum[forum.paradoxplaza.com] that helped a bit with some more confusing achievements
(i know...all of them are confusing, but with the most confusing ones).

Thanks to Cyric for explaining Born Diplomat procedure.