I defeated them with a team of entirely Generation VI pokémon: Meowstic, Gogoat, Hawlucha, Barbaracle, Doublade, and Greninja.
Now, I'm setting out to defeat them again, with a team from each generation. I'm also imposing a few other rules for my teams:
- at least one mega evolution on each team
- at least one Eeveelution, if that generation has any (none for III & V)
- preferably, at least one pokémon that I'm using because I like it, rather than because it's "good"