Shell Smash (Omanyte) @ Eviolite  
Ability: Shell Armor  
Level: 5  
EVs: 76 HP / 196 SpA / 236 Spe  
Modest Nature  
IVs: 0 Atk  
- Shell Smash  
- Muddy Water  
- Ice Beam / Earth Power  
- Protect
Set Details: Shell Smash is a great setup move that doubles both Omanyte's Speed and Special Attack, giving it potential to sweep. Muddy Water is a very good STAB move that hits both opposing Pokemon without touching Omanyte's teammate, and has a nifty chance to decrease opposing Pokemon's accuracy. Ice Beam is usually the best coverage move for Omanyte, allowing it to OHKO Foongus after a Shell Smash boost and to hit Flying types harder. Alternatively, Earth Power is an option to hit Chinchou, which would otherwise wall Omanyte, and Steel types for more damage. Protect is a staple of the Doubles metagame, allowing Omanyte to dodge Fake Out and other threatening attacks to give its teammate a free turn, and to stall out terrain conditions like opposing Tailwind and Trick Room.
This set aims for Omanyte to act as a late-game sweeper or a mid-game wallbreaker. The EV spread maximizes Omanyte's Speed and Special Attack, allowing it to outspeed the entire unboosted metagame after a Shell Smash and to hit as hard as possible. Eviolite gives Omanyte a lot more bulk, allowing it to survive some attacks and to setup more easily. Shell Armor is Omanyte's best option outside of Rain, but Swift Swim can also be used for the off chance the opponent brings rain. Omanyte is most weak to Grass-type attacks, so Pokemon that can check Foongus and Grookey, such as Vullaby, Rufflet or Ponyta, make good partners. Ponyta can also hit Chinchou really hard for Omanyte with High Horsepower, as well as Pawniard, which can be troublesome if it receives a Defiant boost from Muddy Water's accuracy drop. In return, Omanyte can threaten Onix out for the aforementioned teammates. Omanyte also appreciates redirection support to keep it safe when and after it sets up, which can be provided by notably Foongus and Togepi. Foongus also appreciates Omanyte's presence on the field to threaten Ponyta. Finally, Fake Out users, like Mienfoo and Scraggy, can help Omanyte set up by neutralizing an opposing Pokemon for one turn, and the latter can also soften physical attacks directed at Omanyte with Intimidate.

Shell Smash (Omanyte) @ Life Orb 
Ability: Swift Swim  
Level: 5  
EVs: 196 SpA / 76 SpD / 236 Spe  
Modest Nature  
IVs: 29 HP  
- Muddy Water  
- Ice Beam  
- Earth Power  
- Protect
Set Details: Muddy Water is a very powerful spread STAB move, especially in rain and boosted by Life Orb. Ice Beam provides coverage for the Grass types that resist Muddy Water, most notably Foongus. Earth Power is used to hit Chinchou, which is one of the best counters to rain teams. Protect is a staple of the Doubles metagame, allowing Omanyte to give its teammate a free turn, and to stall out terrain conditions like opposing Tailwind and Trick Room.
This set maximizes Omanyte's Speed and Special Attack, allowing it to be a powerful wallbreaker or sweeper in rain. Life Orb improves Omanyte's damage output with all moves drastically. Lower HP IVs are used to reduce the damage Omanyte takes from Life Orb. This Omanyte obviously should be used on rain teams, so teammates that can setup Rain like Riolu and Wingull make good partners. Riolu can also provide redirection support, while Wingull can threaten opposing Grass types like Foongus for Omanyte. Grookey can provide Fake Out support and threaten opposing Water Types for Omanyte, and can also bring it in safely with a slow U-Turn.