Rain Dance Mantine
Mantine @ Life Orb/Heavy Duty Boots
Modest Nature
Swift Swim
252 S. Atk/ 144 Spd / 112 HP (or something slightly more clever idk)
Rain Dance
Hurricane
Hydro Pump/Surf
Roost
Switch in on whatever you like, Rain Dance for the speed boost and wreck shit.
You can tweak this to be more offensive or defensive, but everyone expects Mantine 5o be quite passive when he's got solid offensive capabilities.
Stats let you survive any 2 hits and still outspeed in the Rain. Hurricane OHKOs through Stealth Rock 100% of the time, does minimum like 98%.
Note that Mantine is slower before the Swift Swim, so can Keldeo doesn't get to take advantage of Rain. You can also just Roost through it's Hydro Pump PP.
It's particularly funny when they nope out of the Rain Dance into Toxtricity, who is outsped and one shot by either Hydro Pump or Life Orb Surf even without hazards.