I love Klein but I’ve been wondering how he become one of the strongest players in SAO?

Klein wasn’t a beta tester like Kirito, nor was he properly trained, for a while, by a beta tester, like Asuna.

Klein’s only advantage was the short amount of time Kirito spent, teaching him the basics.

So how did Klein become strong enough to lead a guile that fought on the front lines & suffered 0 casualties?