First gotta say that even though Guts goes Berserk, he when not in berserk mode, is an amazing swordman with incredible reflexes. Been in fights with more than 100 guys and won, even demons. He learns fast and act fast.

His sword is giant, but he can swing it like if it was nothing, incredible fast, faster than anyone can swing their sword. He's got a lot of strong willpower. He's been struggling since birth to stay alive.

Kirito, by far, way far can't defeat Guts, not even in SAO. There is no way, Guts is by far more experience, adaptable, non-human reflexes, smart. And he never gets lucky, he struggles against bad luck to stay alive.

Also, Kirito never experience friendly fire in any fight. While Guts explains very well that he needs to watch as much on his enemies as his allies, cause even a swordmaster can die easily in a war cause of friendly fire.

Plus in SAO it is explained how reflexes were important, and that skill was given to the one that had them better, it'd obviously go for Guts, instead of Kirito, but Guts wouldn't use it as he would prefer a 2H sword.

But stating the obvious, Guts would use his reflexes well enough to defeat the game by himself, unlike Kirito that couldn't do it all alone.