Living bomb + Flamestrike + DIE, INSECTS can deal up to 77 AoE damage to the whole enemy party. If you are lucky with Rag shots on the Living bomb target, possibly even more but its most likely overkill, so its not needed.

Use the 3 mercs on the bench which you want to level, pick a bounty that is low enough to kill everyone on turn 1 and maybe even has loot that you want.

Enjoy farming/leveling!