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For Showdown, I'm not too serious of a player (no account) I have a few teams I try out. Usually get to the mid 1700 ratings and once in a blue moon cap in the low 1800s. Most recently I have something like Mamoswine, Gastrodon, Aegislash, Scizor, Garchomp, and Gardevoir.
Scizor, Gardevoir and Gastrodon are actually my favorites (used them in-game in previous gens), Mamo has become my favorite Ice type, but I'll admit Aegislash and Garchomp are there for their viability.
They work pretty good as anti meta, Rotom never volt switches basically (Gastrodon in particular screws him over) and weakness policy aegislash can be a nasty surprise if they aren't smart and lead with Genesect (and use flamethrower).
Gardevoir is not a mega, in fact I'm not using a mega, it's choice scarfed. Great revenge killer, good typing, can use the last slot for trick or destiny bond. I find it a lot more viable now, Psychic/Fairy means Bug and Dark are neutral (can't take physical too well, but has good special bulk. Can switch into Gengar's Shadow Ball and KO with Psyshock, not guaranteed though if sub, scarfed, predicted.) and leaves it with uncommon weaknesses, one of which is covered offensively. You can run psyshock for utility now that Moonblast is a better STAB. Since you're not a sweeper, you care a lot less about priority. You also outspeed and KO Latios and Garchomp if they aren't scarfed. Once its job is done and the sustained damage you took leaves you easily killed feel free to Destiny Bond an annoying threat. I also find it as a good way to check Heatran, in the best case scenario you trace flash fire, and KO with Focus Blast. Worst case you miss, and hope you can try again (and of course if they have a guaranteed 1HKO with Flash Cannon or something, use Destiny Bond if you can afford losing Garde at that point).