Lol I hate to repeat myself, but I already posted what is, in my opinion, the best defensive partner for Cobalion in the UU cores thread. So I'm just going to plagiarize myself on this one, I hope nobody minds...
Cobalion @ Leftovers / Shuca Berry
Ability: Justified
Shiny: Yes
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Swords Dance
- Close Combat
- Iron Head
- Substitute / Stealth Rock / Magnet Rise
Cobalion is an incredibly threatening sweeper against most playstyles, but it often runs into quite a few problems when it tries to set up. Bulky Water-types (Alomomola, Suicune, Swampert, Slowking), Psychic-types (Cresselia, Reuniclus, Slowking again), and Ghost-types often threaten Cobalion's chances of sweeping, and can even set up on it on some occasions. It also struggles with a number of fat stall cores. Here's where Jellicent swoops in to save the day.
Jellicent @ Leftovers / Colbur Berry
Ability: Water Absorb
EVs: 252 HP / 128 Def / 128 Spe
Bold Nature
IVs: 0 Atk
- Recover
- Hex / Scald
- Will-O-Wisp
- Taunt
Jellicent can't really help Cobalion against powerful Ghost-types, but it can help it with almost everything else that checks Cobalion thanks to its incredibly diverse set of utility moves, typing, and utility. Taunt stops stall and bulky setup sweepers alike, while Will-O-Wisp allows Jellicent to wear down Pokemon and cripple offensive teams. If you choose Hex (which I recommend), bulky Psychic-types don't really stand a chance against you either. Jellicent also checks a few other Pokemon that are troublesome in general but don't necessarily scare Cobalion as much, including CurseLax and opposing Cobalion. Leftovers is a cool item for any defensive Pokemon to have, but Colbur Berry can sometimes be better to prevent you from being Pursuit-trapped, while also giving you a chance to Will-O-Wisp Knock Off users like Krookodile, Mamoswine, Beedrill, etc. This Pokemon takes on a lot of different threats, and it brings lots of utility to the table on top of that.
This core is pretty effective against a large portion of the meta, but it still faces a few obstacles that can prevent it from being truly great. Ghost-types do severe damage to this core, and Chandelure in particular can singlehandedly destroy it, so a Ghost check like Porygon2, Krookodile, or Hydreigon is helpful. Grass-types are also threatening, because neither Jellicent nor Cobalion can really damage them. Finally, despite Jellicent's Fire and Fighting-resistances, you should probably pack additional checks to Pokemon such as Entei, Heracross, Infernape, and Lucario, because Jellicent's physical bulk isn't the best. Florges works well as a teammate because it can absorb most Fighting attacks and heal these two whenever status gets them.