So in the Aqua apartments (to the right of the giant fountain) on the second floor is a girl with level 85 pokemon. Here's the thing - two out of the three have immunities! If you bring a dark type and a ghost type, you can take down her first two pokemon for the low low price of a blackout!
I would recommend having the dark and ghost types have some way to power up (I'm using liepard with hone claws/assurance for alakazam, duskull with will o wisp/night shade for machamp, and I just die against golem).
One note though, Future sight does damage to dark types for whatever reason.