Celesteela @ Psychium Z
Ability: Beast Boost
EVs: 252 Atk / 4 SpA / 252 Spe
Hasty/Lonely Nature
Before i'll go on to explain my set i'll be frank here, i doubt there's a "better" Celesteela set than the standard one just some different spins on the defensive set or fringe offensive sets. With that being said i think i came up with a decent Cleaner here.
Z Gravity provides Celes with a Special attack boost which makes up for the lack of investment and instantly grounds a lot of pokemon that would otherwise wall it without appropriate coverage. Some notable threats include: Rotom-w, opposing Celesteela, Zapdos while still nailing Targets such as Tangrowth, Tapu bulu, Heatran, Magnezone, Kommo-o and Serperior as well as not being stone walled by Lando-t's intimidate as other physical sets would.
However this set does struggle with power issues as it can't reliably 2hko some of the listed targets meaning it will only be able to pick off it's kills once Stealth Rocks are up or the opponents team has been worn down at which point it can overwhelm them with it's coverage.
Side note: Lonely nature is listed as an alternative Option to give Celes better odds to 2hko the likes of defensive rotom in gravity or guarantee ohko defensive tran however it loses the ability to outspeed rotom and zapdos without an Autotomize up.
Note that Stealth rocks will turn a lot of These into reliable 2hkos and ohkos
+1 0 SpA Celesteela Air Slash vs. 248 HP / 252+ SpD Assault Vest Tangrowth: 186-222 (46.1 - 55%) -- 61.3% chance to 2HKO
+1 0 SpA Celesteela Air Slash vs. 248 HP / 252+ SpD Tapu Bulu: 192-228 (55.9 - 66.4%) -- 99.6% chance to 2HKO after Leftovers recovery and Grassy Terrain recovery
252 Atk Celesteela Earthquake vs. 0 HP / 4 Def Heatran: 348-412 (107.7 - 127.5%) -- guaranteed OHKO
252 Atk Celesteela Earthquake vs. 0 HP / 4 Def Magnezone: 324-384 (115.3 - 136.6%) -- guaranteed OHKO
+1 0 SpA Celesteela Air Slash vs. 252 HP / 0 SpD Landorus-Therian: 154-183 (40.3 - 47.9%) -- guaranteed 3HKO after Leftovers recovery
+1 0 SpA Celesteela Air Slash vs. 252 HP / 240+ SpD Kommo-o: 182-216 (51.4 - 61%) -- 91% chance to 2HKO after Leftovers recovery
+1 0 SpA Celesteela Air Slash vs. 56 HP / 0 SpD Serperior: 270-318 (88.5 - 104.2%) -- 25% chance to OHKO after Leftovers recovery
252 Atk Celesteela Earthquake vs. 248 HP / 240+ Def Zapdos: 148-176 (38.6 - 45.9%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Celesteela Earthquake vs. 252 HP / 200+ Def Rotom-Wash: 132-156 (43.4 - 51.3%) -- guaranteed 3HKO after Leftovers recovery
+1 0 SpA Celesteela Air Slash vs. 248 HP / 252+ SpD Assault Vest Tangrowth: 186-222 (46.1 - 55%) -- 61.3% chance to 2HKO
+1 0 SpA Celesteela Air Slash vs. 248 HP / 252+ SpD Tapu Bulu: 192-228 (55.9 - 66.4%) -- 99.6% chance to 2HKO after Leftovers recovery and Grassy Terrain recovery
252 Atk Celesteela Earthquake vs. 0 HP / 4 Def Heatran: 348-412 (107.7 - 127.5%) -- guaranteed OHKO
252 Atk Celesteela Earthquake vs. 0 HP / 4 Def Magnezone: 324-384 (115.3 - 136.6%) -- guaranteed OHKO
+1 0 SpA Celesteela Air Slash vs. 252 HP / 0 SpD Landorus-Therian: 154-183 (40.3 - 47.9%) -- guaranteed 3HKO after Leftovers recovery
+1 0 SpA Celesteela Air Slash vs. 252 HP / 240+ SpD Kommo-o: 182-216 (51.4 - 61%) -- 91% chance to 2HKO after Leftovers recovery
+1 0 SpA Celesteela Air Slash vs. 56 HP / 0 SpD Serperior: 270-318 (88.5 - 104.2%) -- 25% chance to OHKO after Leftovers recovery
252 Atk Celesteela Earthquake vs. 248 HP / 240+ Def Zapdos: 148-176 (38.6 - 45.9%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Celesteela Earthquake vs. 252 HP / 200+ Def Rotom-Wash: 132-156 (43.4 - 51.3%) -- guaranteed 3HKO after Leftovers recovery