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Carbink Diancie @ Leftovers
Ability: Clear Body
EVs: 252 HP / 160 Def / 96 Spe
Impish Nature
- Stealth Rock
- Body Press
- Heal Bell / Moonblast
- Diamond Storm
They do resemble an evolutionary line, don't they? Jokes aside, Diancie synergizes extremely well with Rotom-Mow. They cover each other's weaknesses, with Rotom taking care of pesky Water-, Ground- and Steel-types for the diamond thanks to its typing and Diancie removing Fire-, Bug-, Dragon- and, to a lesser extent, Poison-types out of the way for the lawn mower thanks to its phenomenal defensive typing, stats and ability to break through teams with Diamond Storm + Body Press.
Furthermore, those pokémon together fit both Defog and Stealth Rock on a team within a two-pokémon core and Diancie's access to Heal Bell makes her an absolutely amazing companion to Rotom, who appreciates switching repeatedly into Scalds from the likes of Vaporeon and Mantine. Overall, a very solid defensive structure.
Alternatively to the spread above, a faster version with 252 HP / 132+ Def / 124 Spe can be used to outpace Vaporeon for more chip damage on Vaporeon-weak builds, and an even faster 252 HP / 92+ Def / 164 Spe version can be used to break through Mantine with more ease. It is slightly costy to Diancie's bulk tho, so keep that in mind.