Every generation changed the OU metagame through new battle mechanics, but Forretress is certainly one of the best available spikers, sporting huge defences, wide movepool and Sturdy, which allows him to set at least a level of Stealth Rocks every match. Depending on the Team Preview, i can decide what hazards should be set on the first turns. Gyro Ball is really strong due to the low speed and not-so-bad Attack (eg. Espeon and Gengar get 2HKO'd). The EV spread is pretty easy - specially defensive orientated. Now, Rocky Helmet does 17% damage each contact move it gets. Why is it so helpful? Lots of people use Dragonite as lead, so that they can get boosts while the lead sets hazards and/or sponge attacks with Multiscale. Basically i set two-three Spikes layers while it dances, then he FirePunches, i hold on with Sturdy, Rocky Helmet breaks Multiscale, and Gyro Ball is 1HKO. This strategy works 99% of the time, and can help Volcarona a lot due to the lack of HP Ice
Scarf Gengar replaced my Scarf Latios, as i wasn't brave enough to spam Draco Meteors. Trick stops the nowadays-common Blissey, and allows Scrafy to get some random dances. Gengar also switches in Ground- and Fighting-type attacks to help the rest of my team, and can get U-Turns without many troubles. Shadow Ball is the main move, Focus Blast completes the coverage and hits Tyranitar and Steel-types, Hidden Power Ice is for dragons and Gliscor
Later
Volcarona is my final cleaner. I decided to use it after i've seen how great Forretress is at spinning and taking Dragonites out. Quiver Dance, then proceed to sweep, 2-3 kills per match guaranteed. Fiery Dance is extremely strong, and has also a nice 50% of raising the SAtk by 1. Bug Buzz is the other STAB, and gives a great coverage (for the likes of Latias). Hidden Power Ground hits Heatran SO HARD. If you choose HP Ice, you can't do anything at all, and you're forced to switch even after 2-3 dances. It's also helpful against Steel-types on rain teams as it's supereffective. Leftovers is mainly to heal the Sandstorm damage, but also to get back some HP lost due to Stealth Rock
Later
Used it because i though it had Moxie, and i'm too lazy to change it on the teambuilder. I decided to abuse Dual Chop to kill any Focus Sash / Multiscale abuser, and it's way better than Outrage. The Speed is quite low (97 only), but it's really fast after a Dragon Dance. Earthquake is for coverage, hitting Steel-types (especially Heatran). Rotom-W is completely screwed due to Mold Breaker, Earthquake is 1HKO if somebody forgets Levitate is useless. I don't remember what's the EV spread for, but i don't care. Taunt shuts Ferrothorn / Blissey down, so they can't cripple me while i 2HKO. I'm thinking Outrage > Taunt, is it any worthy?