That's a cool team, however, some common threats give it trouble. Most Rotom-W can be VERY annoying to this team because Mamoswine, your only Electric resist (or immunity in your case), is OHKOed by Hydro Pump so it can really use Volt Switch over and over, weakening your team a lot. This opens you to more powerful sweepers who will then clean your team easily and since you have no Spinner, you'll play with a weak team most of the time. Rain Calm Mind Jirachi could be a big issue too because whenever it gets a Sub it can then get multiple boosts and it's very difficult for your team to deal with it, as most of it easily loses in 1v1 to either Water Pulse or Thunder. This Jirachi version really got a boost in popularity with BW2 and I see it quite often. Since they have the same kind of coverage you can cover both with one change, which would be swapping Dragonite for Specially Defensive Celebi. Celebi annihilates Rotom-W in 1v1 and it can even take Will-O-Wisp thanks to Natural Cure. While it can't do really a lot to Jirachi, it can use Perish Song while tanking its attacks to phaze it out, and if it's a last pokemon situation it lets you win in 3 turns, which is quite convenient. Dragonite's defensive coverage is a bit redundant will Celebi's so I think that this change is the one that leaves you open to the less threats.
About your Gastrodon problem, if you take my suggestion, you won't have this problem anymore as Celebi is one of the best Gastrodon switch-ins so it allows you to keep HP Ice on Rotom-W which is always cool since Latios tends to switch into it often, allowing you to hit it hard with a bit of prediction.
If you're worried about Rain Volcarona, you should definetly try out a different Politoed set. If you swap it to a Specially Defensive set with Leftovers, you can tank 2 boosted max SpA Hurricanes or Bug Buzz and strike it back with a powerful boosted Surf for the 2HKO. While it's kind of a shaky check, it's always better that what your team is trying to do to it at the moment. Besides, I don't think your current Politoed set is accomplishing a lot and now it can really support your team with something else than Drizzle.
Here are the sets you should use:
Celebi and Politoed
EVs: 252 HP / 4 SpA / 252 SpD
EVs: 252 HP / 4 Def / 252 SpA