All castles in Heroes 3.

Galthran is given Air magic in 90% of cases. A normal player will always take it. So, speeding him up and casting a single Titan spell (or whatever everyone is using to attack) won't be particularly difficult...

but if there's no other option...???

As an alternative, I think either boosting a strong mage (Explosion + Earth Sphere), or using a Vulnerability Medal (here you can try Berserk, Blindness, and all sorts of debuffs), or (I'll be testing soon) using more Behemoths than enhanced units (so they ignore defense, which is decent due to Galthran's specialty).