Play as Inferno.

In this confrontation, everything depends on the artifacts and the initial placement of heroes and troops on the scale at the beginning of the final battle! If you don't manage to block Isabel's Knights at the start, you're doomed! And the fact that only Nimue can counter Haven in the late game is a fact.

But no one can. Although, I remember we played with open companion heroes. I really enjoyed playing as the Warlord; it's actually possible to win with him. Nimue is only viable if you have the Creature Ability (CA). This is already in the late game, level 25 and beyond. My games usually happen at levels 21-24... Sorry...