These achievements are very tedious in my opinion, but I found a way to make them less time consuming. I'm not saying this is the best way to this, but this is what worked for me.
You can do a lot of custom battles like I did.

Unfortunately the 10k, 100k and 250k kill achievements doesn't "stack", so we will need to kill 360k units in total. I did some math after unlocking the 100k achievement and counted the battles I completed.

Following this guide should give you 7023 kills per battle.
100.000 / 7023 = 14.2. So about 15 battles for the second achievement.
250.000 / 7023 = 35.6. So about 36 battles should do the trick for the last achievement.

It took me 1 hour to get the conqueror achievement after I had unlocked the slaughter achievement.

https://steamproxy.com/sharedfiles/filedetails/?id=1290403994

I used this mod, for the extra unit counts.
Zombies goes from 40 to 60 per unit.
But more importantly Helstorm Rocket Battery goes from 1 to 2 per unit.
The way I did this, only works with this mod, else the zombies overrun your rocket batteries.

You want to play as the Empire against 3 armies of Vampire Counts.
The battle location should be a nice flat land with no trees in the way. I used Galbaraz.

Set the battle difficulty to easy, and large armies with custom funds of 100.000 ( max amount ).


Choose a random lord for yourself, and then fill your army with 39 Helstorm Rocket Batteries and fully upgrade them.


The 3 undead armies should consist of zombies only. I used 3 Strigoi Ghoul Kings as their lords, and removed all their skills.

When you enter the battle, your artillery should be lined up just fine, and the undead should be more or less blobbed up. I did encounter a few battles where the undead didn't blob up, though the result was more or less the same.


Next up you just start the battle, and for extra speed, pop the 4x fast forward button.
You don't even have to do anything, the artillery will do it for you.
It only takes a few minutes per battle.


That's all you've gotta do.
Good luck and happy grinding. :-)