Or just bait then onto throwing it active. Here's the thing, it has to be active for the wall, meanng they aren't attacking with the GX Ninetails. Mini-tails takes 5 hits to kill 1 Metagross, a banded one takes 3. Meaning if they keep it active, you can easily set up for free. Maybe play a Guzma here or there, but either way, that Ninetails is not doing anything to Metagross GX+Max Potion. Sure you could use Metang, but honestly, I would just put Metagross up front and wall it right back. The only way Ninetails works as a wall is if it's the only thing on the board. Have a Metang sitting on bench and just Guzma around Ninetails BUS, if they leave that Pokemon as their sole one on board, win the game with Metang. It's a bad strategy to play BUS Ninetails into Metagross, because it's useless.Although those non-gx metagross are rotating (ancient origins), even leaving a metang unevolved with 3 energy will 1 shot a baby ninetales with metal weakness (80x2)
You just have to notice the matchup and try to leave one unevolved