I reviewed the last 2 issues of the series today.
Pretty sure he's referring to the following, most of which actually happens in issue #6:
Source: Cosmic Adventures in the 8th Grade #6
Circa: 2009
If memory serves, Emperor Joker had all of Mxyzptlk's power.
But Supergirl in this series had so much power at the point depicted
(commanding much of the energy of a multiverse and all)
that Mxy felt compelled to flee to his own dimension to escape her.
Mxy would handily roll in most any OTHER scenario;
I gotta go with Supergirl on this one.
