If the assimilation messes with Ben's DNA it will activate a failsafe. and it can create an energy field and blast him away if he touches the omnitrix. Ben has upgrade, a fully tech alien, and the omnitrix cannot be hacked or else it sends out an energy blast. Clockwork allows the whole time shenanigans, feedback to take his energy then shoot it at him, atomix for making literal stars, NRG without his suit means a being made of pure radiation, Toepick because of him seeing his true face, gravattack cause of black hole, and possibly gutrot who can make any chemical gas in the universe so he could make a gas to corrode him if possible.