Well, what can Mewtwo do? Genesect resists Mewtwo's STAB attack and is SE against its psychic typing.
It's movie-wise... not game-wise.
But the producers would still have to follow the in-game mechanics to some extent. Anyways, I guess when we go with some movie logic, Genesect would get the upper hand in the beginning. Mewtwo tries its psychic attacks directly at Genesect, gets surprised that it doesn't work and then gets hit by one of its bug attacks. Mewtwo gets beat up. And then using its IQ of 10,000, he psychically searches the vicinity for a nuclear missile and telekinetically throws it to Genesect along with a Fire Blast. End of..
Genesect uses U-turn and switches with Dark Plate Arceus . --- Mmm... Pies...
They could have Genesect beat Mewtwo as a plot point for the movie...
Or they could appeal to older fans and have Mewtwo win.
I'm on the side of tie, with a slight leaning for Mewtwo due to lolplot armor. --- http://24.media.tumblr.com/tumblr_mbn8e2Vsjl1qibib0o2_r2_500.gif Robo MC Fun and the Funky Three Plus One are coming to your town!
They fight for ages then they become friends and fly away and Ash continues his travels and never mentions it ever again.
The battle will go for 10 years... Never-ending epic battle... One will win in a stupid way... Ash never ages. --- "Definition: 'Love' is making a shot to the knees of a target 120 kilometers away using an Aratech sniper rifle with a tri-light scope." -HK-47