Boros DOES hit Saitama so hard that he impacts the moon at a significant fraction of lightspeed (it's seconds at most, light takes about 1.2 seconds to hit the moon). That's a LOT of force. He's also definitely fast enough to just relax and kill all the Godzillas he wants. He will attack hundreds, thousand of times, so Big G would need to be able to flat-out no-sell the guy. And I don't think that he can.