How to defeat Deleb on "Old Friends"?

That's clear. It's not entirely clear how to get to the center when it's guarded by mages or archers!

Well, the Academy has an advantage here (Necropolis to a lesser extent). And you don't necessarily have to use the same hero in a single game: for example, use Nazira to break through mages and archers, and combine heroes. Also, look for the right hero in the tavern, chests for gold, not for experience (although it all depends on the specific situation).
There is no standard scheme or recipe to achieve victory. You have to improvise...