Custom battle vs AI
Options: Large Armies, Unlimited Funds, Very Hard battle difficulty (+Leadership for AI)
Map: Choose one of the subterranean maps
Player Faction: Empire
Player Army: Hellstorm Rocket Batteries (Max rank)
AI Faction: Vampire Counts
AI Army: Zombies (Max rank - for leadership), Necromancer lord (He stays behind so artillery does not focus/kill him)
Also make sure your unit size is on Ultra in graphics settings.
The reason i buff leadership is because i don't know if units dying from crumbling count as kills. I don't think they do. You should still get about 4k kills per game though and can do it semi-afk.
You can do this whilst doing the 25 MP battles achievement. Make room with password and then put vs AI.