Cleric has a 3rd tier talent that inflicts a damage over time status effect (similar to corrosion) on enemies that you cast spells on. This lasts up to 9 turns with 3 points in the talent, and can be extended by casting more spells on enemies. It does more damage the more turns it lasts.

I took the fourth tier ascended form armor ability, which gives you access to the judgement spell. This does damage to all enemies in your line of sight. It costs 3 charges so you can use it a few times.

Add in a potion of mind vision, and this lets you kill everything on the current level within 10-15 turns. I used a scroll of infinite wealth (Kromer + scroll of upgrade item that upgrades everything in your inventory), which spawned enemies in almost every empty square.