Celebi @ Weakness Policy
Ability: Natural Cure
EVs: 252 HP / 156 Def / 80 SpD / 20 Spe
Bold Nature
- Recover
- Calm Mind
- Giga Drain
- Baton Pass
Diancie-Mega @ Diancite
Ability: Magic Bounce
EVs: 4 Def / 252 SpA / 252 Spe
Naive Nature
- Moonblast
- Diamond Storm
- Hidden Power [Fire]
- Rock Polish
The idea for this core came from
this Pokemon World Cup match, so props to the dude there.
As for the details: Outside of covering its weakness to water and ground, Celebi's role here is to pass boosts to Mega Diancie. This is done via CM and the item Weakness Policy, which gives you +2 in attack and special attack after being damaged by a super effective move. Celebi has many weaknesses and is quite bulky, so it's usually able to tank most SE moves and pass the boosts afterwards. For example, consider some of the things Cele tends to draw in: Torn-T and Talon. Celebi can stomach a Hurricane, U-turn, and Brave Bird from them and then pass to Mega Diancie, who does a good job threatening those Pokemon and other threats disliked by Celebi (like Tran and Tar). Once in with the boosts, Mega Diancie can possibly RP up and go for a sweep.
Mega Diancie is a good recipient of Celebi's boosts for a few reasons. As mentioned earlier, it's able to threaten a good number of Celebi's common switch ins, namely flying and fire types. It's blessed with Magic Bounce to protect itself against annoying status like paralysis and poison. Mega Diancie also has access to a speed boosting move to fully take advantage of the boosts being passed, and respectively abuses the +2 in SpA and Atk with Moonblast and Diamond Storm. Further, Mega Diancie becomes rather bulky after being passed some CMs, being able to take attacks like Keld's Scald (as shown in the replay) and RP up after. Although its resistance to Talon's and Sharp's priority helps, it's still vulnerable to priority from Pokemon like Mega Sciz. This is why the core is normally paired with Keld, who is another good receiver of Celebi's boosts.
Anyway, BP isn't a very consistent strategy so the idea behind this core and a good number of others won't always work in practice. The core has its flaws and isn't perfect, but it's still fun to mess around with and can have devastating results when pulled off successfully as the replay showed.