In order to beat Metagross you need to out speed them as having three metagross is usually their win condition so parallel is good against the deck. The other way to win is if they over commit their energies to the field because everything in the deck has a high retreat cost they then struggle to retreat, trapping them in the active spot.
The best fire pokemon to take out metagross is usually salazzle, For two energies Salazzle does 110 or 50X each prize you have taken which with a choice kills metagross. I've heard the new pyroar is somewhat interesting and probably would do well against Metagross as well.
If your deck is just Fire, Grass, and/or water, as Parallel city reduces the 110 from salazzle-gx's heat blast to 90(before weakness, resistance, and other factors such as choice band) if you try and limit bench first(by putting the "three pokemon" side of the stadium face-up on your opposing side, and in the process, putting the "reduce damage from all Grass, Fire, or Water Pokémon by 20 before weakness and resistance"), that strategy is a double-edged sword that won't OHKO Metagross, with or without choice band:
Heat Blast+Choice Band w/ parallel city(-20 GFW side facing you):
110(base)+30(choice band)-20(parallel city)*2(weakness)=240 damage
250(Metagross-GX max HP)-240(result from above)=10 damage left on Metagross-GX...
^ Right now, you could Po Town/Espeon-EX, but come rotation time, that will go out the standard window, and sadly, there will be no good options for devolving the bench remaining in standard, but you can use upcoming Celestial Storm(CST) Blaziken-GX in tandem w/ Magcargo CST, Rayquaza GRI, Commandment Shrine CST, and Zoroark-GX to put the autoloss(?) pressure on Metagross!