Counter-heroes in duel mode (3.1).

I can't seem to find a hero who can handle Karli.

I think Ravenna is the best choice. You need to lose to Karli with her, but you have to try :) and follow these simple rules/tips:
1) Don't attack units under the "intangibility" rune.
2) Naturally, use Chaos magic to attack those who don't have a rune protecting them from the elements.
3) In the first round, it would be good to use a ring of cold to kill the artillery and also hit high-level monsters (ideally, Jarl and Drakosha).
4) Don't expose yourself to battle rage, especially from Jarl and Dragons. Maximum 2 units next to each other.
5) And the overall tactic is a defensive style of play with careful troop movement or counterattacks, counting on a prolonged battle in which Karli's chances tend towards "0".
P.S. And the hero herself (Ravenna) is not particularly scary, so feel free to put her against Karli, Jalib Deidra, and Axel.
P.S.P.S. And in my opinion, an interesting match is Ravenna against Razazak.