How To Defeat Kevin
To defeat Kevin destroy the objects that he absorbs. If you complete this he wont be able to regain health. Then use Humongisaur's attacks to finish him off. Be aware that his attacks do more damage when he's metal or stone.
Time Out Combo
First you put on two player mode, then you and your partner do the same exact combo move at the same time next to each other. Then you might see a green light after that your omnitrix will time out and you will turn back into Ben 10.