It is possible that a Level 1 Ratatta can defeat a Level 100 Mewtwo by using the F.E.A.R startegy.
I tell you how to perform it:
Teach your Rattata the moves Endeavor and Quick Attack. And give it the Focus Sash to hold.
On this first move, use Endeavor. The opponent will obviously attack first. But the Focus Sash will save it at 1 HP. Rattata will use Endeavor and it will bring the opponent to 1 HP (that's what Endeavor does; bring your opponent's HP down to your's). Then, the nxt round, use Quick Attack. This move allows your pokemon to move first and attack the opponent. Your opponent wil faint and you and your Level 1 Rattata (or which pokemon you wish) will win.
The only possible way this stategy could not work is if you are facing a Ghost Type since Quick Attack will not effect it.