First off, I'm not cheap, so I'm not gonna get a lvl 100 Mewtwo. If you seriously need one then you're more of a noob than me.
Secondly, I win every fight I can through type advantage, so I beat every trainer even though I have weaker guys on my team. I beat the last gym using lvl 35's.
Thirdly, I cannot stand using the Vs Seeker, it's too much hassle going everywhere again. I much preferred Emerald's PokéNav. Much more efficient.
I may as well do the rounds with the Vs Seeker. It'll take another week or three, but it might help.