I wouldn't change any of the factions, but I would gladly change some creature stats. Most likely, I would increase the initiative of Minotaurs and, instead of regeneration, give Hydras some ability like "charge" (like Rakshasas have). Then these two types of slow units could actually be useful in battle.

Or maybe not, I would still replace Minotaurs with something flying, because when attacking castles, you have to rely only on magic and dragons. By the way, why were Gargoyles assigned to the Wizards? I think they would look more organic in the Dungeon, for example, with the Dark Elves. If you add more health and damage to them, they would become an excellent Tier 3 unit.