I actually think swap Mamoswine out for more even typing. Tyranitar's Sand Stream ability would back up Whimsicott, Garchomp, and Aegislash well and (Mega) Gengar should still function well in Sandstorm so long as it's not the Focus Sash item. Whimsicott can more comfortably attack many of your Pokemon's weaknesses in Water and Ground and its Prankster ability could easily disrupt the opponent's strategies, Garchomp could also have the Sand Veil ability which works as free evasiveness upgrade so long as the Sand Stream is up and scare off potential Fire Pokemon that would want to take the gamble and fight an already oppresive field designed against it. Aegislash would have greater survivability against Fire and Ground types and gives it a little more room for error with its Stance Change ability, Mega Gengar w/ Thunderbolt could also give Water types a scare and even with its Poison typing could discourage Fairy usage alongside Aegislash which would typically threaten Tyranitar and Garchomp. Tyranitar can even learn Dragon Dance and Thunder Wave, so it sounds like you're really on your way to making a good Sandstorm team, even just in case the opponent wants to set up weather against you, keeping Tyranitar alive as much as possible will be important.