Answers to Lesser horrors...

[Executor]

That all depends on how many mages and horrors there are.
One lightning or frozen heart won't kill them, several will.
However several lightning or frozen heart won't kill several horrors since they won't be aiming at the same horror and they have a life drain attack.
I'd say horror mark is the best if multiple horrors are being sent,
however I won't be sending horrors against your large armies with mages, I have something much better planned for that.