Mr.lol, Gengar needs STAB Shadow Ball. I'm assuming you are switching around focus blast and hp-fire. Shadow ball is the best option for the myriad psychics that show up, such as Mewtwo. And yes, Whimsicott with Prankster Ability may be used in any weather (although it needs its leftovers to cancel out damage from sand or the rarely seen hail).
Obviously this gen's ubers is pretty much weather spamming. If Kyogre is the summoner, I would obviously go the top three swift swimmers (Ludicolo, Kabutops, Kingdra) or maybe someone like Lanturn. Pikachu with Light Ball might be an option, at least if given the speed boosts first, as it is really frail compared to the ubers, and is under the base 90 speed tier, the same as Groudon, Kyogre and company. If it's Groudon, the obvious options are the Chlorophyll sweepers. Yes I am talking to you, Venusaur! Also obvious are the big fire attackers, such as Volcarona and Darmanitan. What if it's another weather? First thing I think of is the Tyranitar to Excadrill combo, especially if Excadrill carries X-scissor for those random psychics. And Jolly to out-speed stuff like Scarf Chomp.