Strange had been sitting underneath a tree, writing something in that little notebook of his when Susan's attempts to fly caught his eye. He spends about thirty seconds or so watching before realizing oh, those wings are flapping and he's not getting anywhere, that explains it. Standing up, Strange stretches as he walks over towards Susan.

Logically, those tiny little wings shouldn't be able to let him fly. However, Strange has weird mirror eyes and they all grew gills that one time, logic doesn't matter. If Susan thinks he can fly then of course he can fly! He probably just needs some help.

"Perhaps you need a bit of a head start? I can call up some wind, maybe that'll help you stay in the air!"

There is no way this will not end badly, or at least end up with both involved looking more ridiculous.