The whole point is Predator is walking power house. Armed or not, you can`t just beat him with strenght. That was very strongly pointed out in first film and unless you are an ancient super weapon hellish monster like Alien, you won`t stand a chance. Still, Celtic was beating the shit out of Grind in close combat! He was close to win! Pred vs Pred? I guess without weapon it would look more like MMA fight then some kung-fu fantasy and the fight from Predators was something like that.