Mr. X is the tyrant from RE2, kind of looks like Nemesis in a way since he's massive and wears a trenchcoat too.
But anywho, compared to all the other tyrant models, Nemesis seems like he's almost indestructable. He survived his own rocket launcher's explosion and then being thrown into a dissolving pool, only coming out stronger each time (though he did consume a tyrant's body for his last mutation). He then survives several shots of a rail gun, finally put down by several revolver shots to the head.
You also have to remember Nemesis is intelligent enough to use weapons, so if had his rocket launcher he'd easily be able to win.