I absolutely loved this episode and the whole fight…but I also completely agree with you. This whole show has been building up Viltrumites as an OP omega-level threat, which made me feel genuine fear for Mark this entire episode, which was brilliant! But then, suddenly this baby Viltrumite who two years ago was basically a normal human and only got his powers like a year ago, was able to beat up and almost kill a Viltrumite veteran who has probably been alive for thousands of years. And the explanation is just “you’ve been holding back all this time, don’t do that anymore”?

It’s exactly the kind of bullshit you’d see in a Shonen anime (which is why I hate most Shonen).