Take into account that Aliens have razor sharp teeth, claws, speared tail, acid blood, they can walk on walls, jump and leap far and are very fast!
But, Predators are very strong, can withstand a lot of damage, very intelligent, excellent stealth, can perform surgery on itself, fast, good jumpers and leapers, have no fear and would rather die than accept defeat.
Then there is the common Marine smart, well armored, tracking devices, quick thinking, well trained and hopefully smart enough to figure out that without weapons the Predator is no threat the him.
