DISCLAIMER: I have not tested this build. It is risky. Take with a grain of Shoal Salt.
NATURE: Naive (+Speed, -SpDef)
EV SPREAD: 12 HP / 172 ATK / 72 SA / 252 SPD
ITEM: Weakness Policy
--Rock Slide/Stone Edge
--Ice Beam or Focus Blast
Come in with full HP when you can predict a super-effective attack and Shell Smash on the same turn to trigger Weakness Policy for +4 attacking stats and +2 Speed.
Go with Aqua Jet for STAB priority. Rock Slide for STAB and flinch (or use in doubles), or Stone Edge for more brutal STAB.
(If you're feeling really crazy, you can opt for Ancientpower and cross your fingers for more stat boosts.)
[NOTES ON THIRD MOVE]
Water/Rock/Fighting is not resisted by any Pokémon in the game.
(Neutral vs 376, Super-effective vs 423)
Water/Rock/Ice is only resisted by Poliwrath, Keldeo, and Empoleon.
(Neutral vs 386, Super-effective vs 409)
- Naive nature, the EVs listed above, and a +2 Speed boost puts Carracosta at a decent 358 Speed as well as a massive 1180 Attack and 880 Special Attack (all stats at Lv100). You don't have to be a mixed sweeper, but you have the ability to do so.
- Use Jolly if you don't care about special attacks for a bit more special bulk.
- Consider supporting with Sticky Web or Follow Me, or Tailwind in doubles.
- Other move options include Waterfall, Scald, Hydro Pump, Knock Off, Crunch, Earthquake, Iron Head, or Giga Impact for devastation.
You could also opt for Solid Rock (or even Swift Swim on a rain team), but your options for triggering Weakness Policy become slimmer and require better prediction.
Definitely risky, but the payoff is a "turtle" powerhouse.