I like to use Weezing against bosses. It's usually tanky enough to live an attack then I can lower the boss's offenses with Memento. Neutralizing Gas lets it bypass the immunity to stat drops Gardevoir normally gets from its ability
I honestly kinda doubt that would happen since gorilla tactics isn't that great for doubles and her field weakens fire moves. Eiscue on the other hand would be a pain in the ass without special moves and definitely something I see her using
That would make sense, I think Jan confirmed that we'll get the mega ring in V13 through story events, and there are already a few mega stones available in sidequests
She was pretty easy for me thanks to the Psychic Terrain. I had my Simple Swoobat use calm mind for +4/+4 and just swept everything with stored power. To get around the trick room I just put it on my Sableye and kept using trick room until her Aromatisse fainted