I used Gastrodon here. Storm Drain (almost due to the brief ability wipe) completely ignores Hydro Pump, it can run Clear Smog which will wipe the initial Double Team, and Chilling Water will help out the entire team by nerfing its use of Gunk Shot and Night Slash (the latter of which is helpful with all the Slowbros running around). From there you could run a physical build with Earthquake and Curse, or a support build with Mud Slap, which considering it runs lower accuracy moves was a boon for avoiding some of its nastier attacks, and Recover. I personally used the latter and the team we had was able to get the clear pretty comfortably.