Who can beat this master of powerstacking?

True but once you amass enough power you can win against a person who's smarter than you.

my list of possibles:

Leach ...... no honestly
Xavier at his prime.... simply mind wipes him. as he has no psionic deffence
Morphus
Lucifer
Timothy hunter
Dazzler

and who can forget squirrel girl ( who wins by default)

the unbeing