"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

I did the same; Ornella was level 30, and Arantir was level 17, even though I didn't level him up, but I managed to complete the game.

Added 5 minutes later
You need to buy 3 heroes, and Ornella is the 4th. You can pass through the portal if you killed the spirit killers; if not, you can't use the portal. You can only pass through one portal.