You're assuming they hit the Predator. Mac hit the Predator by accident, he didn't even know what he was shooting at.
And I'm not just talking multiplayer, the singleplayer Predators do pretty welll in their campaigns against many enemies. Even the enemy Predators in the singleplayer campaigns take a LOT of punishment before they go out.
The big thing in 'Predator' was that no matter what gear the humans had, it wasn't terribly effective. What mattered was that Arnold outsmarted the Predator and beat it at its own game. Having high-tech gear doesn't really matter if the Predator can outsmart it. Arnold took down a Predator by using mud, a home-made bow and arrow, and a big log. He didn't do it because those implements are inherently good, he did it because he was able to outsmart the Predator and then use them effectively.
It's why the player-character Predators in the AvP games can take down Marines armed with high-tech weapons, as well.