>>16142237First of all, why are we even having Gardevoir waste time with Shadow Ball when STAB SE Psychic is much stronger? Why are we even arguing about a one-on-one match that, by the way, will be won by Gengar if both sides play optimally? Why are we even debating the relative worth of Ghost and Fairy when both are unusually blessed by the type chart?
To address your scenario, you are going about this the wrong way. The Gengar player brings in the non-Mega to scare away something that it will absolutely kill, let's say a CB Close Combat user for shit and giggles, and Mega Evolves and uses Perish Song as you switch in Gardevoir on the next turn. Gengar hides behind Protect and Substitute for the next two turns, with M-Garde possibly retaliating with a Hyper Voice at some point if it predicts the Sub turn correctly, but ultimately dies when M-Gengar switches out on the last turn because Shadow Tag is a bitch like that.
Running Focus Band only works 10% of the time, that's bullshit and you know it, and Sash only works if you can keep the Rocks and the Sand away. Shadow Ball will kill if you don't run Sash or Kasib Berry, and if the M-Gengar user predicts that you'll switch in Gardevoir specifically and not something that isn't weak to Ghost, they'll just Shadow Ball on the switch and outspeed and kill on the next turn.
You could also try Scarf Gardevoir, but if you aren't using Mega Gardevoir you're already making a suboptimal choice, and Scarf Gardevoir is especially bad.
