Finally, an Extreme Battle Event that isn’t trivially easy…
It’s obvious what they were going for here. Roserade spams Swift to counter the obvious f2p choice that is Mewtwo. In order to get around this, we have to be a bit more clever. The general idea is: Let Roserade use Grassy Terrain, Sleep right side, Sleep middle, sync takes out left side, then just keep attacking.
Gengar can be replaced with any unit with Sleep. Troublemaker is nice to have, but not required. Lunatone is a good support for Mewtwo, she gives +4 Sp.Atk and +4 Sp.Def. Mewtwo is the most important unit, ideally fully gridded with CS2 and theme skills. As for the grid, I think going for Endurance is nice just in case you take more damage thant expected. You can certainly go for a pure power build, with Brainteaser + Smarty-Pants, but it will depend more on RNG.
You’re not guaranteed sync crits and it’s not the end of the world if you don’t get them. If left side survives, you can use Hypnosis on it to get the turn skip. If right side survives, he uses Giga Drain which is less threatening than Swift. It will ultimately come down to RNG, but you can still improvise your way out of each situation.
As much as I dislike sleep strats, it feels less scummy when they have condition nullifcation, so that you can only sleep each enemy poke once.