At first glance, before Corrin posts it, I've naturally thinking about P2 as a check. But we're at a fun place so goodbye classics and welcome uncommon stuff.
Cresselia @ Rocky Helmet / Sitrus Berry / Wiki Berry
Ability: Levitate
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Ice Beam
- Moonlight
- Psychic / Lunar Dance
- Lunar Dance / TWave / Trick Room / Toxic
Offensive calcs
0 SpA Cresselia Ice Beam vs. 252 HP / 0 SpD Landorus-T: 132-156 (67.3 - 79.5%) -- guaranteed 2HKO
0 SpA Cresselia Ice Beam vs. 4 HP / 0 SpD Landorus-T: 132-156 (80 - 94.5%) -- guaranteed 2HKO
0 SpA Cresselia Ice Beam vs. 252 HP / 0 SpD Assault Vest Landorus-T: 88-108 (44.8 - 55.1%) -- 60.2% chance to 2HKO
Defensive calcs
Landorus-T Earthquake vs. Levitate Cresselia: 0-0 (0 - 0%) -- lololololololololol
252+ Atk Landorus-T U-turn vs. 252 HP / 252+ Def Cresselia: 62-74 (27.3 - 32.5%) -- 55.2% chance to 4HKO after Leftovers recovery
252+ Atk Choice Band Landorus-T U-turn vs. 252 HP / 252+ Def Cresselia: 90-108 (39.6 - 47.5%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Landorus-T Knock Off (97.5 BP) vs. 252 HP / 252+ Def Cresselia: 84-100 (37 - 44%) -- 99.8% chance to 3HKO after Leftovers recovery
252+ Atk Landorus-T Supersonic Skystrike (175 BP) vs. 252 HP / 252+ Def Cresselia: 114-135 (50.2 - 59.4%)
I prefer P2 overall but Cres is very relevant against Lando-T and can offer a free switch to a teammate with Lunar Dance.