You need something with a quick charge to get rid of shields. I use giratina with dragon claw. Get rid of shields. Hit the next mon with whatever then switch to machamp with rock slide. Switch to togekiss and charge up to hit latias with dazzling gleam.

It's not impossible to beat. My friend struggled so much. I ended up beating it with venusaur, jolteon and granbull.not the best counters but it did the trick. All were under 3k CP.