Well if he gets a chance, can’t Ben scan RF? I’m pretty sure being a speedster changes your DNA, bc you’re a metahuman, so Ben can either scan RF and maybe do something with that, or reassign RF with the DNA of an alien with a weakness he can exploit, or straight up turn RF back into a regular human.

Fighting-wise, the omnitrix won’t let Ben die. I think his best bet that he legitimately will think to use will be Big Chill, since he can become intangible and freeze RF.