If you're an offensive player and want to breeze through the challenges, Blackguard is excellent. Veterans make good grunt attackers, and Leaders are capable of blocking the majority of enemies, while Disintegrate can eliminate whatever they can't. Add that with Drain and some Thugs at the start, and you are virtually undefeatable.
I don't know what lag people are complaining about, on the latest version of Firefox, this game runs very smoothly. Although I was a little unsettled at how easy this hard badge was. I actually got the medium achievement significantly later than the hard one.
Corruptor: "I here them coming. You six, go cram into that tiny room away from everything, they'll never suspect a trap there."
Lich: "But sir, we're your best fighters, shouldn't we be deployed on the frontline?"
Corruptor: "Frontline? What nonsense are you spewing? All our soldiers will be hiding in unorganized groups distributed randomly throughout this level. All except for you, Gargoyle, you alone will be defending the passage to my chamber."
Gargoyle: "Er... So I assume you'll back me up if the fight begins to go ill?"
Corruptor: "No, I will in fact not be confronting them until all of our forces get completely massacred."
Lich: "What?! Sir, I don't see the strategic value in-"
Corruptor: "We don't have time to discuss this, they'll be here any minute! Now go, and don't forget to unlock all our armories and treasure chests on your way out. They might need some new weapons after dirtying them hacking through all of you."
The hero upgrade system forces the length of this game on you. Even with every unit fully upgraded, without your hero mastering every skill, which appears to happen someone around LEVEL 100 (yeah), you cannot be expected to win quickly.
You're all part of a devious and cruel experiment