Celebi doesn't have room for Perish Song (you want to have Baton Pass, Recover, Thunder Wave, and damage move), and that's literally one thing you'd use it for.
252+ Atk Huge Power Azumarill Knock Off (97.5 BP) vs. 252 HP / 0 Def Celebi: 258-304 (63.8 - 75.2%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Celebi Giga Drain vs. 16 HP / 240 SpD Assault Vest Azumarill: 102-120 (29.5 - 34.7%) -- 4.2% chance to 3HKO
You take over 60% from Knock Off, lose your Lefties, and are forced to use Recover as Azumarill switches out to whatever Grass resist your opponent has. Or a T-Tar.
Amoonguss has Regenerator and can take an HP Flying. Azumarill is a check since it resists everything and last time I checked Scald is considered hax for the definition of a check/counter so it technically is a counter anyway.
You give a shit because it negates Lefties and are forced to switch out.
Never said Alomomola won, I just said it didn't care.
Slowbro can hit you and switch out because Regenerator is awesome.
Celebi @ Leftovers
Ability: Natural Cure
EVs: 252 HP / 236 Def / 20 Spe
Bold Nature
IVs: 0 Atk
- Perish Song
- Giga Drain
- Heal Bell
- Recover
The set I use and it beats every water type you mentioned, all while giving Heal Bell support (which was the dealbreaker for me, why I used Celebi over other grass types). If you run a more offensive team you could probably use Thunder Wave over Heal Bell since that type of support benefits offense more. I skipped Baton Pass because Celebi is physically defensive, so it can actually take on some Pursuit Tyranitar.
Physically defensive is also why I can take on Azumarill. Recover off the first boosted Knock Off, and after that it does 38% max.
Amoonguss takes a tiny bit more from HP Flying regardless of EV spread so that's kind of irrelevant?
252 SpA Choice Specs Keldeo Hidden Power Flying vs. 252 HP / 220+ SpD Celebi: 146-172 (36.1 - 42.5%)
252 SpA Choice Specs Keldeo Hidden Power Flying vs. 252 HP / 0 SpD Celebi: 196-232 (48.5 - 57.4%)
252 SpA Choice Specs Keldeo Hidden Power Flying vs. 252 HP / 252+ SpD Amoonguss: 162-192 (37.5 - 44.4%)
252 SpA Choice Specs Keldeo Hidden Power Flying vs. 252 HP / 4 SpD Amoonguss: 234-276 (54.1 - 63.8%)
Meanwhile "hax" in the definition of checks and counters is about the hax you can't really do anything about. Almost no Weavile counter can do anything about Ice Punch's freeze, so we disregard it. But you can do something about Keldeo's Scald by running a counter that doesn't mind burn. So Azumarill is an ass Keldeo counter 30% of the Scalds where Celebi / Amoonguss is not, you can't deny that. Also, it's technically "hax" to
not get burned if Azumarill switches into Scald twice (only 49% to avoid burn both times), and it has no recovery to switch in all day anyways, forcing Azumarill to paired with a second good answer to Keldeo.
About Rotom-W, I really don't give a shit about lefties being negated for like two turns. If that damage meant anything, Rotom-W would be better off just staying in and Hydro Pumping because that does more than 13% per turn. And I'm not "forced" to switch out of the burn, I simply have the option. Like if Amoonguss, another good Rotom-W counter, gets burned by it, it's burned whether it stays in or switches out. But Celebi has the opportunity to get rid of its burn by switching and now it's "forced" to like that's a bad thing?
Not like Celebi was walling whole teams, it was gonna switch out anyways, and it's one of the few defensive Rotom-W counters that can double switch it because Rotom-W risks eating a Giga Drain if it Volt Switches.
Also if you don't care about Celebi recovering on your Alomomola, setting up on it, and/or taking away half it's health, good for you. Most people care about that.
Slowbro hits Celebi and switches out, meanwhile, Celebi recovers all that damage on the switch out turn, assuming it even predicted the Celebi switch in. It's not getting anywhere.