Sliggoo @ Eviolite  
Ability: Sap Sipper  
EVs: 252 HP / 4 Def / 252 SpD  
Careful Nature  
- Curse  
- Rest  
- Outrage
- Sleep Talk / Iron Tail

Notes: Sliggoo is an extremely one dimensional Pokemon; it walls opponents specially and it sets up a Curse. Curse is it main boosting move, allowing you to increase your defense in tandem with Eviolite. It also increases your attack, transforming Sliggoo into an extremely threatening Pokemon. Switching into Pokemon such as Servine, Roselia, and Wartortle are free set-up opportunities, due to your natural bulk and your ability, Sap Sipper. Rest allows Sliggoo to negate any burns or poisons on it, and restores your HP allowing you to either check powerful special attackers such as Kadabra and Electabuzz, or letting you continue to boost with Curse. Outrage is the main STAB attack, hitting extremely hard after a few Curse boosts. Sleep Talk can be ran in the last slot, allowing you to potentially boost and attack while asleep, or Iron Tail can be used, allowing you to hit Fairy Types such as Togetic and Clefairy.
Checks and Counters: Marill can be considered Sliggoo's biggest counter, being able to take neutral damage from Iron Tail, while trapping and eliminating with Whirlpool and Perish Song.
If you're not running Iron Tail, then Fairy types can completely shut down Sliggoo, either to boost their stats or control hazards. Certain Bulk Up users such as Vigoroth and Gurdurr are a problem for Sliggoo, as the person who wins the 1v1 is the person who can land a Critical Hit first. 
Other Options: Due to Sliggo's one dimentional nature, it's not beneficial to run any other options.