Well, he seems to have quite a few things going against him here. Like A. His special attack isn't too great and B. He has no physical water moves...
But hey, let's see what we can do.
Torterra (M) @ Life Orb
Trait: Overgrow
EVs: 4 HP / 252 SAtk / 252 Spd
Modest Nature (+SAtk, -Atk)
-
Rock Polish-
Earth Power-
Giga Drain-
Hidden Power [Water]He actually get's over 400 speed after a single rock polish, so he's not too shabby.
Earth Power + Giga Drain for STAB. Giga Drain also get's HP back from orb.
HP Water is rain boosted coverage.
Torterra (F) @ LeftoversTrait: Overgrow
EVs: 252 HP / 252 Atk / 4 SDef
Adamant Nature (+Atk, -SAtk)
-
Leech Seed-
Stealth Rock-
Earthquake-
Wood HammerThis set focuses more on the fact that Politoed can't learn stealth rock, meaning you won't have hazards coming from your lead.
He sets hazards and leech seed. With Leech Seed + Leftovers recovery, he can safely use Wood hammer. While if he really needs some recovery, earthquake is always open too.
He doesn't have to worry as much about flying types due to the fact that you probably are packing an electric type that can OHKO any flying types with STAB Thunder.
Overall, he has a resistance to fire through the rain, which is awesome considering he still has the STAB Earthquake to wipe fire types out. He has the HP Water option. And can lay down Stealth Rocks, which is a plus considering Politoed can't. While he's still slightly lacking, he's better than I expected.
Rating for the rain -- 4/10