A friend of mine had a nigh unbeatable "necromancer" deck that was freaking gigantic. It had something like 300 cards in it. No one could topple this monstrosity. Basically his strategy involved going through his deck and sending most of the really strong creatures to his graveyard, and then bringing them into play from his graveyard.

I'm happy to say I was the first of our small group to conquer it. I built a 40 card direct damage deck. The only creatures I had were 1/1 creatures with haste. I loaded it with fireballs, lightning bolts, and other similar things. I basically pinged him to death with small amounts of damage before he could get his first creature in play.