I've found as long as you have dcent ranged units and two blockers, defending is the best way to go. I have a demon hunter and a longbowman, which isn't that much, but the blockers in front ensure the two ranged units kill everything easy, even against the firestorm groups.
If I decided to make a new party, I'd probably do 2 longbows, 2 assasins or grey knights, and probably a demon hunter(the piercing damage is amazing), then go defense all the way.
I found that if you do all of your units berserkers and upgrade them even more to the berserk class, you never lose because you keep regaining health and so yeah...