Naruto Thread (Spoilers For Sure)

Itachi had to practice a lot with Izanami.

Really, your first example that you come up with is a character who spends most of the manga offscreen (Ample time to learn new things) and was introduced as one of the strongest people in existence to begin with, alongside the fact that itachi never lost to anyone anyway, so you could always argue he never went all out.

If you can't spot the difference between that and a clear case of Deus ex machina, well...