Never mention a song of ice and fire!! Martins writing style is pure shit, and his story is going no where. I hope he dies before he finishes the series.
Cain survived those encounters he didn't kick their asses.
Space marines are the best of the best, but now a single space marine isn't good enough compared to some figures. Even with being genetically superior with the best weapons against something like a big tyranid, they are screwed.