Steal his Gravity Shield to block against the brunt of the attack (Firemasters). However, this will not shield you forever, because he has Momentum. Fire Storm and send your Otys in to eat creatures, or just Fire Storm twice with a Bonewall or a Graveyard up. If you have Lobotomizer or Mind Flayer, you can get rid of Momentum. Be wary not to let the Otys touch your creatures; don't play a Graveyard if the opponent has Otys out. Remember, you can eat your own creatures to swell your own Oty to a large enough size to eat all of Graviton's creatures. Mutant spam should be able to finish him off.