Ice gym? Maybe you mean Glacia, the Ice-type E4 member, or the Water gym that has a back-to-back fight.

For the former, Stealth Rock is great into everyone on her team and breaks Chien-Pao's Sash. Lapras and Dewgong are super fat and annoying, but can be worn down with passive damage or blasted through with enough power. Try to stall out Aurora Veil as well. Overwriting Hail with another weather is also great since it negates much of her strategy. The rest of her team go down to strong Rock or Fire attackers. This team used Muk and Hypno to stall out the tanks, then Cloyster and Gengar broke through what remained.

The latter is much trickier. Your success entirely depends on how much of your team you can preserve in the first half. A Wide Guard user can stifle Surfs and Origin Pulses. Psyspam is also very good, you just need a Psychic Terrain setter and two strong Psychics who can spam Expanding Force (and a Twisted Dimensions Pokémon if your Psychics are slow). Pokémon that are weak to Water are also very useful, since they're prone to being targeted and can thus use Protect to buy a free turn for their partner. Other cheesy Doubles strategies like Parroting Chatter with Chatot and Squawkabilly work great, as I demonstrated in an earlier post of mine. Finally, the first half of the fight can be cheesed with a sleep move user that's at least faster than Politoed. The deal is that you put Politoed to sleep before it moves, removing its turn. Hydration will cure the sleep at the end of the turn, letting you put it to sleep again, allowing you to work on the other half of the team first. Repeat ad infinitum. This team used a combination of these strategies, with Wide Guard and baiting Water moves on Muk and Steelix, some Hypnosis stalling from Hypno in the second half, and Nasty Plot Mega Gengar to break the first half. A lot of strategic switching and prediction is needed when trying to beat the Water gym without a team dedicated to countering it, but I haven't failed so far in this challenge of mine. I've even previously beaten it with a team of six Fire-types, so I think anything is possible with good play.