Ice and Fire

+ Mass Dispel only reduces stats by half, i.e., 2 Mass Dispels = 1 Chain Lightning in terms of turns.

Chain Lightning will kill 15 Priests (and even then, there might be a deviation), while Mass Dispel will remove the entire stack from the battle.

Added 1 minute ago

But that's not the main point anyway.
The Imps are still weaklings – there are 400 Horned Beasts left, as well as 200 Cerberi – I gave them to the second hero. And a few more Cave Trolls won't make much difference, in my opinion.

Although I should probably take care of that. Now I'm on the offensive – I defeated a garrison with 95 Jarls, losing 65 Bears, but they weren't needed anyway.