Xuen is only a Wild God. If a simple fel-infused Orc chieftain can kill one it isn't really all that impressive a feat. Not really a good way to measure Lei Shen's power. Beating a Titan keeper on the other hand is rather impressive, but still not all that amazing.
But who knows, maybe one day he'd have figured out how to unlock all of Aman'thul's power. But we killed him before that happened. In the end I guess he never reached his full potential.