Very cool team you have here so props for it! However, I have some advice to give you so let's start with the rate. First of all, as you already said, your team has some problem against Rotom-W because you haven't switch into it and Rotom-W can beat Politoed, Moltres and Tentacruel pretty easily. Therefore, I suggest you to use Rest and Chesto Berry replacing Power Whip and Leftovers on Ferrothorn. Rest + Chesto Berry allows Ferrothorn to avoid Rotom-W's Will-O-Wisp and that can help you a lot. I think also that a recovery move can be useful for Ferrothorn because it lacks Leech Seed so it can die quite quickly. Then, I see that Mamoswine can be problematic for your team because it beats Garchomp, Thundurus-T, Tentacruel easily and nobody can switch into it. To solve this weakness, I'd move 76 EVs from SDef to Spe on Tentacruel. It has already 20 EVs in Spe and 96 EVs allow to outspeed any Mamoswine, expect the Choice Scarf one of course and kill it with Scald under rain. Finally, another threat for this team can be Volcarona. Volcarona can setup easily on Ferrothorn and then smash your team, especially if sun is up. I think that Stone Edge > Aqua Tail on Garchomp can help you a lot against Volcarona anyway this is optional, if you prefer Aqua Tail, you can keep it but I suggest you to play carefully if you find Volcarona in the opponent's team.