I don't know if this has been said but I figured I'd share the strategy I used to beat the 3 co-op levels. It's boring but it's really simple.
All you have to do is run around the enemies castle. If you're a cleric with dash or a mage with wind trap, it makes it easier to avoid getting killed.
What this does is it takes at least 4-5 bots out of the fight so your teammates will have an easier time destroying the towers. But once the Big Tower is attacked it won't work, you could probably run around the top of the castle but at this point you should have enough time to spam the tower until you win.
And co-op is a good place to get the "kill everyone" and "kill everyone and destroy the big tower" quests.