I tested the team on PO and I found a couple of things that might help the team.
1) I gave Cresselia protect instead of ice beam. It allowed me to live longer and got me through a couple of tight spots. Although ice beam was helpful, I always prefer a spot for protect.
2) I gave Garchomp dragon gem instead of yache berry. I prefer dragon gem because it was more useful. Yache berry could only keep garchomp alive half the time and isn't used every game. With dragon gem, I can use it every game and it gave me the extra damage to get more KO's in.
3) I gave Volcarona fire gem instead of focus sash. With all the weather users around, volcarona was going to sufer residual damage before focus sash could be used. The main rock slide users were garchomp and tyranitar, garchomp usually being paired with tyranitar. Volcarona has a solid bulk so besides rock slide, not many things are capable of KOing it in one move.
4) I replaced tentacruel with two pokemon, ludicolo or toxicroak which were both good. I know ludicolo didn't work for you, but I prefered it over tentacruel because of leech seed which gives it an adavantage in hp recovery. Toxicroak essentially resists most rain teams by itself. With drain punch or posion jab, it is a great counter to abomasnow. (I prefer drain punch for the hp.) Ludicolo only has ice beam against toxicroak which hits it with neutral damage. It has access to fake out which was useful for volcarona to set up.
5) I gave Volcarona quiver Dance instead of hidden power. With two fake out users, it could set up very easily when with them on the first turn. I barely used hidden power and garchomp resists Volcarona's chandelure weakness well.