Easiest way to get this is in skirmish with the following set up
Army 1 (You):
Clan - Dracul
Lord - Famed Spellsword (LVL 10)
Item, Weapon - Mace of the Berserker Ring - Ring of the Seven Sons (For both ring slots)
Units - 2 Acolytes (more if you are having trouble, just to heal your Lord)
The rest doesn't matter
Army 2 (CPU):
Clan - Nosfernus
Lord - Nosfernus Pawn (LVL 1) <---- Just don't select any level and it will be automatically lvl 1
Units - 6 Skeleton Warriors (They respawn when they die and are easily another kill since they aren't full health. You can get 2 kills per unit).
Start the match, don't move for the first turn until the very end by waiting, move forward and let everyone surround you. Kill everyone after they have been damaged once. Kill everyone using your lord. Overkill is triggered when you kill an enemy and gives you an additional attack. You should easily kill 4+ in one turn.