AP galio is quite weak against duelist/divers
From play him and against him after the patch, AP Galio is like a glass cannon. He's strong against immobile melees, but anyone with a bit of mobility would be a nightmare for him. The shift in his damage from base to ratio, leaves him with a weaker early game where his harass doesn't hurt as much and he isn't able to clear out the wave as fast. Early game is where you want to exploit Galio's weakness. In terms of build
is a really good item against him as it provides %damage reduction against his %hp tick which is major by mid-late game.
While AP galio hits hard before hotfix, it quite easy to kill him after his W duration is down. I honestly found Ahri more annoying to play against than Galio.