odin was unable to help bill by himself and couldn't understand what the surfer was doing to bill. thanos's cosmic power served him well against odin -- odin was simply the more powerful of the 2.
mr. mxy has been said to both use 5th dimensional 'science' AND magic. at the highest levels they are impossible to tell apart. lt has waved off strange's spells.
if you're saying JUST power cosmic, (as opposed to general 'cosmic power' ala eternity and lt, etc . .. ) and using galactus as the all-encompassing standard, then you CAN have more powerful magic wielders, but galactus has stalemated mephisto in his realm AND aggamotto in his realm so at similar levels NIETHER appears to have the advantage power-wise.
they are really opposite each other -- magic breaks physical law to do things that are 'impossible', while pc uses the laws of the physical universe to make almost EVERYTHING possible.
has anyone come up with a feat that is accomplishable by one but not the other (when comparing similar levels)?