The most challenging campaign.

It's not necessary at all.

I used Isabel as a support hero everywhere except in the 1st campaign. And in this and the 5th mission, I generally transformed all the knight troops into undead for Markel. Only at the beginning of the mission did she sail into the sea for some goodies. Her starting army is enough to break through the half-submerged ships. And then I took the troops with a support hero and sent them for processing into undead. She was always running around next to Markel.