Should I evolve this or wait for a better one?
I’d wait. Except you “need” it now for a raid, which i don’t think.
Larvitar is back at the front of the rock rockets. You can likely find a few more before the end of the TM event to try for a better one. Even then, with Ttar’s elite move being it’s fast move, it’s not even absolutely necessary to have frustration TM’d away - if you have the extra candy/dust to get a 2nd move that is.
As ever, the stock shadow advice - even a terrible IV shadow will outperform a hundo non-shadow. If nothing else comes along by the time Cresselia gets here, it’ll do fine.