Basically, he knows the game in and out so well from his time playing the game, he perfects some bosses later on, by himself. While no damage runs of a video game might not sound like the most impressive thing IRL, imagine seeing it first hand, in person. Perfectly I-framing / dodging attacks at the exact right moment / etc etc. The manga won't get there for a while, though. Got an annoying character getting introduced next arc.