Volcanion... I don't think so. Vika/Bulu is faster.
Gardevoir GX maybe, but with Nature's Judgement's energy discarding it is still difficult for Gardevoir to OHKO Bulu.
I think Metagross is the best Vika/Bulu counter (almost never a OHKO unless you play 2 Flying Flip's with Tapu Koko and then start with Nature's Judgement + Choice band and Max potions make that even harder), maybe Alolan Ninetales (GX and non-GX).
And indeed Garbotoxin of course, but that could be Field Blower'ed away or Guzma'd. If a Bulu player has like 3 turns without Garbotoxin, that's already 6 extra energy... Garbodor is OHKO without discarding energy, Espeon/Drampa are OHKO with (Espeon) or even without choice band (Drampa).