I got this problem in Stone fortress with 2 bosses.
I think my level was around 29/30 and theirs is around 27.
Especially the necromancer who is with Isabelle.
He used exploding arrow + poison and I was killed in 2 shots + 1 second (for the poison).
The second one was with the general you meet for the first time in one of the interior dungeon.
In both cases I could finally kill them by hiding and/or taking back the teleporter, rest, and back to the fight.
And spamming all my spells.
Playing pure mage helped.
Imarion
