Supergirl bodies Flash in the neutral. It's literally like one of the most lopsided neutrals in the game IMO, Flash has a below average neutral (especially according to Honeybee) and Supergirl is one of the best characters at controlling space (and if it weren't for Starfire she might be #1). If you're getting opened up nonstop by Flash you're probably playing the MU wrong. Stay fullscreen, laser to control space, use b12 to punish jumps, and he struggles far more than most to deal with her float, both when she uses it defensively and offensively. He has to commit to everything he does, make him miss and whiff punish. Don't let him jump in on you. His d2 is one of his best options so don't air dash if you're not getting to his far side. Flash can spend 60 seconds to back you into the corner and then you hit him with a single f23-teleport and he's back to square one. He's Flash, in order to win: Slow it down.
IMO...
EDIT: Oh and delay wakeup, he really struggles if he can't time your getups.