Batman vs. Mr. Mxyzptlk
First thread in a while here.
Anyway, both are in-character, and Mxy is just trying to prank/annoy Batman, not hurt or kill him. Batman wins if he can trick him into saying his name backwards.
However, in order to make it more of a challenge, beforehand Mxy erases all of Batman's knowledge of Mxy and the 5th dimension, so Batman doesn't know his weakness and has to figure it out for himself. He also can't contact Superman or anyone else to find out the weakness.