Perfect prediction implies that the Pokémon will always do the best move against you. Here, it means that Plasm will attack Azelf in the switch with Thunderbolt or Sludge Wave (because they 2HKO it). Yet the LO damage just secures an OHKO on Plasmanta, so Azelf wins.
So, with Miasmaw out of game, Volcanion is the one to look at, and Stratagem surely don't counter it. Still, note that Neutralizing Gas would nullify Levitate, therefore you should take Earthquake into account as well.
Tapu Koko @ Choice Specs
Ability: Electric Surge
EVs: 252 SpA / 4 SpD / 252 Spe
- Dazzling Gleam
- Grass Knot
Koko outspeeds it and gets the kill
Edit: minor fixes on writing error in calcs