in all honesty none of the aliens in AVP would've been able to stand toe to toe with the original Giger Drone or one of Stan Winston's Warriors. It seems like over time the xenomorph's have been portrayed as little more than savage beasts, pretty much the equivalent of a raptor. You can see it in almost all AVP merchandise & products, the games & comics in particular, where a single predator is able to mow down groups of xenos with relative ease on their own, when in fact, most times a single alien would make a decent match for a pred.
Grid was getting his @ss hammered throughout the entire fight, sure he got some clawmarks into Celtic's mask, but pfft, what's that really gonna do? I also think that Anderson's bias got in the way while he was shooting that scene, as there is no possible way that Grid could have taken all of that punishment & simply stood up without so much as a scratch on him.
Overall, in my opinion Celtic was working harder for the kill, wheras in most cases Grid was just trying to survive the brutal beatings that were being dished out (although he made a couple of good sneak attacks during the fight).
If it were not fot Celtic's cockyness & overconfidence towards the end of the fight, he would have definantly been the victor on that occasion.