Warhammer Quest 2: The End Times

Start any white or red dungeon ( not random ambush encounter ) and have 4 of the same type as your party. I had 4 blood knights. I picked an easy encounter with rats and finished dungeon. Unlocks on the mission complete screen.

Presume it will work with 4 dark elf sorceresses, 4 bretonian knights, 4 ogres etc.