Um, why not just use the right support? Groudon - Makes Thunder less accurate, and if Tenta is using Ice Beam, Knock off, Rapid Spin, Toxic Spikes, it wont worry about Sun weakening surf.
Worried about Black Sludge? Use Giratina-O (keeps its item and blocks Rapid Spin), and have Arceus on your team and since Tentacruel is weak to Thunder and Earthquake why not use a Grass Arceus? It cannot lose its item, and resists Tentacruel's weaknesses.
If you don't want Grass Arceus, keep in mind that Giratina-O already resists Tentacruel's weaknesses, and Groudon is Immune to Thunder, so Arceus can be any type you want. You can also have a Steel Arceus since it can help take Dragon hits and Recover.
So, next is having a Wish supporter... How about Latias? It can handle Kyogre, and support your team, plus it's HP Fire, if it uses it, is boosted by Drought, so it can handle Scizor and Forretress who may try and Rapid Spin or Pursuit. Latias also resists ALL of Tentacruel's weaknesses!
So Lead Groudon, Tentacruel, Giratina-o, Latias, Arceus [Grass or Steel], and finally decide what your team will need most... perhaps a physical wall that can pseudo haze? Lugia.