You would need to be a lot higher than the level 50's to beat the final boss with just Gambits. He's actually a bit tricky cause he likes to hide behind physical/magical palings and spam his ultimate attacks. high 60's I could imagine if you have most of the end-game gear.

I'm tempted to try it actually but I'm currently working on taking down Omega Mark XII who wouldn't be so much of a pain if the path to get to him wasn't littered with high level undead, not to mention the damn Phon Coast Trophy Hunt quest has bit me in the ass again and these Zombie hordes keep having a high level Rare Game Spirit spawn with them. He loves spamming death and the zombies love hitting me with cloister breath...

I may need to level a bit more to take on Omega as well as abuse the Cat Ear Hoods.