To be fair, I've only watched most of the anime. Manga Genos might be awesome or something. I'm just saying that of what I've seen, he's a bit (read: a lot)on the overconfident side.
Even with just anime stuff, though, there's a big gap between tanking Krillin's slow-ass disc just because he thinks he could, and... anything he's done in the anime thus far. Yeah, he let Mosquito Girl charge up her power when he shouldn't have - but he didn't let her beat the absolute goddamn shit out of him afterwards. She'd just become too powerful for him to do anything about at that point, except potentially self-destruct. Then he turned his back on Sea King when he shouldn't have, (though, to be fair, it wasn't
terribly unreasonable to assume SK was totally fuckin' dead at that point), and lost an arm because SK was fast as fuck enough to get in a free attack from it.
Much as I enjoy making fun of Genos's unfortunate failures, those two instances are pretty much his
only actual blunders in the anime. The rest of his fights are either clean sweeps, like Armored Gorilla, or lost simply due to an overwhelming power imbalance, like Kabuto and Tatsumaki.
Also, does anyone else think that they'd be very unlikely to fight at all if they met? I mean, Krillin typically only fights for his friends or for exhibition purposes, and Genos usually only seems to fight to test himself, protect others, or pursue his cyborg nemesis. Only the first would apply to a fight with Krillin, and Krillin would be nowhere as good a choice for that as Saitama anyway, who Genos has access to on a regular basis.
Nah, they'd both be totally down to spar. Genos just needs it to be presented to him as a form of training, while Krillin, as a career martial artist, would already see it as such. All it'd take is for each of their respective sensei's to set it up.
Oh yeah, we have a legit issue. How much time wasting inaction is there in 1PM for fights? Because Krillin would have a problem fighting without breaks for monologues or flashbacks or excessive time powering up. I mean, a lack of endurance is gonna be a major factor here.
OPM fights include very little wasted time by comparison, to the extent that that sort of thing is sometimes initiated by one party just so that the other party can jump in and cut it short for laughs.
Edit: Genos is
occasionally an exception, though. He kinda weirdly back-and-forths between being straight down to business and monologue-ing, sometimes.