Gordon Freemans's crowbar could only scratch Master Chief's armor, but if he were to the the AR2's secondary shot, it'll either disintegrate Master Chief, or bounce off his armor, he could use the Magnesiums from HL2: Ep 2 and stick it to Master Chief and use his pistol to blow it up, or he could just blow the Chief up with the RPG
But then again, Master Chief could stick a grenade to Gordon Freeman, or use any of his weapons to kill him, or he could just use the Energy Sword to slice him in half or something
If you use real physics, it would take Gordon plenty of time to dent Chief's armor with a bunch of bullets, or if you were to use Halo's physics, Gordon could easily kill Chief with the SMG, AR2, or any of his other weapons, but can't Gordon use his Gravity Gun to throw a heavy object at Chief? He can't pick him up with it though, he's way to heavy
That's all i could think of