I haven't used it in a long time, and I only had mixed success when using it before (probably due to me being a horrible battler at the time instead of merely bad), but I've always thought a Wring Out/Brine Omastar could work. Something like...
Omastar @ Life Orb
Modest / EVs: 72 HP / 252 Sp. Atk / 184 Speed
Ability: Swift Swim
- Wring Out
- Brine
- Hidden Power Grass
- Ancientpower/Ice Beam
72 HP gives it 299 HP for minimum Life Orb damage while 184 Speed lets it outspeed Jolly Swellow in the rain. Wring Out is 121 power when the enemy is at full health, while Brine is 130 power (not including STAB or rain!) when they dip below half health. That Water/Normal combo is only resisted by Shedinja in UU, and Ancientpower takes care of it. HP Grass helps deal with the Water Absorbers while Ice Beam is an option for Grass types/Altaria if Shedinja doesn't scare you. Grumpig takes Wring Out pretty well, but as soon as it gets below half health, Brine rips it apart.