Experienced something weird in HSTR P1 with my CLS team

That just means he has more protection, what keeps it working is R2 dispelling the defense down each time they crit, which for rebels is always, once he dies, that stops, Nihilus stops dying, hence if you try it without R2 you will get almost no damage I would guess. That's just my experience though, if it works without R2 next time, I don't know.