Watching the video where you kept sending the Great Drakes into Reimu's city and losing them.
Aside from those heroes just being high level and generally OP, what was going on is, the wizard has Guardian so all their units had additional +1 To Defend and Hit, then they used Prayer which added another one so overall they had 2 of each. The Chaos Warrior has Armor Piercing, so does the lighting from the fortress, so they can both hurt the drake quite effectively, too.
Aside from those heroes just being high level and generally OP, what was going on is, the wizard has Guardian so all their units had additional +1 To Defend and Hit, then they used Prayer which added another one so overall they had 2 of each. The Chaos Warrior has Armor Piercing, so does the lighting from the fortress, so they can both hurt the drake quite effectively, too.