Celebi@Leftovers
Ability: Natural Cure
EVs: 252 HP / 56 SpA / 184 SpD / 16 Spe
Calm Nature
- Nasty Plot
- Giga Drain
- HP Fire/HP Ice/Earth Power/Ancient Power
- Thunder Wave/Substitute/Baton Pass
Celebi is a pokemon that I think has been consistently underrated. While it's easy to understand why Tangrowth and Amoongus are the defensive Grass types of choice (Regenerator really is just such a good ability), Celebi has always had the tools necessary at its disposal to be a viable team member, with several advantages over its competition no less. For one, Tangrowth and Amoongus are SLOW. Celebi has a very nice 100 speed tier that allows it to perform as a pivot without necessarily being forced to take two hits consecutively. Also, Natural Cure has its own merits as an ability. It makes it harder to wear down from Scald burns or Toxic. Celebi is one of the most consistent checks to Keldeo for this reason and should not be overlooked, as well as the ubiquitous Rotom-W. And while it doesn't have Regenerator, it does have access to Recover if you are so inclined. In practice, you may find Celebi's bulk surprising compared to Tangrowth thanks to Leftovers, since Tangrowth usually wears Assault Vest or even Rocky Helmet.
Another benefit, more specifically over Amoongus, is its lack of passivity. A solid 100 SpA with access to both Calm Mind and Nasty Plot. I find that a bulky spread that takes advantage of a +2 Giga Drain makes Celebi impressively difficult to break through if given the chance. But it's also a terrific lure that just decimates presumptive safe switch ins with your coverage of choice.
+2 56 SpA Celebi Hidden Power Fire vs. 248 HP / 200 SpD Mega Scizor: 304-360 (88.6 - 104.9%) -- guaranteed OHKO after Stealth Rock
+2 56 SpA Celebi Hidden Power Fire vs. 252 HP / 168 SpD Ferrothorn: 280-332 (79.5 - 94.3%) -- 6.3% chance to OHKO after Stealth Rock
+2 56 SpA Celebi Earth Power vs. 252 HP / 188+ SpD Heatran: 400-472 (103.6 - 122.2%) -- guaranteed OHKO
+2 56 SpA Celebi Hidden Power Ice vs. 252 HP / 8 SpD Landorus-T: 436-516 (114.1 - 135%) -- guaranteed OHKO
+2 56 SpA Celebi Hidden Power Ice vs. 240 HP / 0 SpD Garchomp: 420-496 (100.7 - 118.9%) -- guaranteed OHKO
+2 56 SpA Celebi Giga Drain vs. 92 HP / 0 SpD Azumarill: 410-486 (112.6 - 133.5%) -- guaranteed OHKO
+2 56 SpA Celebi Earth Power vs. 0 HP / 0 SpD Bisharp: 366-432 (135 - 159.4%) -- guaranteed OHKO
+2 56 SpA Celebi Hidden Power Fire vs. 0 HP / 0 SpD Bisharp: 246-290 (90.7 - 107%) -- 75% chance to OHKO after Stealth Rock
+2 56 SpA Celebi Hidden Power Ice vs. 0 HP / 0 SpD Latios: 170-200 (56.8 - 66.8%) -- guaranteed 2HKO after Stealth Rock
+2 56 SpA Celebi Ancient Power vs. 248 HP / 252+ SpD Talonflame: 332-392 (92.4 - 109.1%) -- guaranteed OHKO after Stealth Rock
I think this is where Celebi shines. Celebi is deceptively bulky that outside of hyper offensive super effective attacks, it can survive many coverage or neutral STAB moves comfortable and retaliate with a coverage of your own, or predict a switch to something like Tornadus-T or Weavile and nail them with a Thunder Wave.
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 252 HP / 0 Def Celebi: 159-187 (39.3 - 46.2%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Life Orb Bisharp Pursuit vs. 252 HP / 0 Def Celebi: 166-198 (41 - 49%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Life Orb Bisharp Sucker Punch vs. 252 HP / 0 Def Celebi: 330-393 (81.6 - 97.2%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Life Orb Latios Draco Meteor vs. 252 HP / 184+ SpD Celebi: 211-250 (52.2 - 61.8%) -- 97.7% chance to 2HKO after Leftovers recovery
252+ Atk Talonflame Acrobatics (110 BP) vs. 252 HP / 0 Def Celebi: 288-342 (71.2 - 84.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
232 SpA Pixilate Mega Gardevoir Hyper Voice vs. 252 HP / 184+ SpD Celebi: 172-204 (42.5 - 50.4%) -- 41% chance to 2HKO after Stealth Rock and Leftovers recovery
252 SpA Life Orb Gengar Shadow Ball vs. 252 HP / 184+ SpD Celebi: 260-307 (64.3 - 75.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 SpA Mega Manectric Flamethrower vs. 252 HP / 184+ SpD Celebi: 154-182 (38.1 - 45%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
252 Atk Tyranitar Crunch vs. 252 HP / 0 Def Celebi: 270-318 (66.8 - 78.7%) -- guaranteed 2HKO after Stealth Rock, sandstorm damage, and Leftovers recovery
252+ Atk Choice Band Tyranitar Pursuit vs. 252 HP / 0 Def Celebi: 218-260 (53.9 - 64.3%) -- guaranteed 2HKO after Stealth Rock, sandstorm damage, and Leftovers recovery
Tyranitar can honestly has a difficult time Pursuit trapping Celebi, Scarf variants in particular. A +2 Giga Drain will heal a significant portion off of a Choice Scarf Crunch, after which you can switch safely. Similarly, CB Tyranitar Pursuit only does about 60% if you don't switch out, again allowing for massive healing or paralysis shenanigans. And then there's always the option of baton passing out to safety! In fact, Keldeo is an AMAZING partner for Celebi who can paralyze fast opponents, BP a +2 while Keldeo easily takes a resisted Pursuit, and then you're free to set up a safe Substitute or in certain circumstances, a +2 Scarf set could feasibly sweep a team.
As for the spread, if you use Substitute max HP allows for 101 Subs that Chansey can't break, easily allowing you to set up to +6. This set does well against Stall as even without any boosts Unaware Quagsire hates Giga Drain. 56 SpA EVs is what allows for the guaranteed OHKO on bulky Mega Scizor after SR with HP Fire, which I felt was necessary. I feel SpD investment is the way to go over physically defensive because it really helps with Draco Meteors and Moonblasts, as Celebi has a pretty good match up against Magic Guard Clefable, even if it has Flamethrower, since Celebi can boost faster and both heal and attack at the same time with Giga Drain, while Clefable can't.
Celebi is very customizable for team purposes though. If you don't feel like Baton Pass or have other means of taking out various Steel types than also consider Celebi's access to Stealth Rock and Healing Wish. With more support moves you can up the defensive investment a bit but I think it's worth it to be a consideration over Amoongus. But Healing Wish is a great and limited move that can easily turn the tide of battle by resuscitating your wallbreaker. Healing Wish sets pair particularly well with Charizard X. Celebi can live and slowdown things like Latios and remove Heatran with Earth Power so Char-X can stick to its STABs, and naturally counters Quagsire all in one. And don't forget Healing Wish dodges Pursuit instead of Baton Pass. So really, Celebi has a LOT of utility in one slot.
http://replay.pokemonshowdown.com/ou-231674833
Only semi-decent replay I could find on hand. Again it's old, but Celebi manages to get rocks up twice while paralyzing two opponents.