Galio support is an amazing pick. Just start your Q and get Spellthief for that extra AP and proc thunderlords with your Q every time you poke. You will do great damage.
When the ADC is low because of the poke, E through the minion wave, hit your passive, W, Q. If he's not dead by this point youre doing something wrong or their support is a god.
Build RoA, finish your Eye of the watchers, and get Abyssal. Now that we are midgame going into lategame, build Iceborn for the armor.
If you did not get fed/win lane, build Iceborn first.
Not only is this a great build, but its also really fun to play.