First set of calcs are without critical hits.
252+ Atk Life Orb Kingdra Waterfall vs. 252 HP / 252+ Def Cresselia in rain: 144-172 (32.43 - 38.73%) -- 4.3% chance to 3HKO
252+ Atk Bisharp Night Slash vs. 252 HP / 252+ Def Cresselia: 156-186 (35.13 - 41.89%) -- 83.52% chance to 3HKO
252 SpA Alakazam Shadow Ball vs. 252 HP / 4 SpD Cresselia: 144-170 (32.43 - 38.28%) -- 2.27% chance to 3HKO
252 SpA Life Orb Gengar Shadow Ball vs. 252 HP / 4 SpD Cresselia: 273-322 (61.48 - 72.52%) -- guaranteed 2HKO
252+ Atk Choice Band Technician Scizor Bug Bite vs. 252 HP / 252+ Def Cresselia: 308-366 (69.36 - 82.43%) -- guaranteed 2HKO
252+ SpA Choice Specs Hydreigon Draco Meteor vs. 252 HP / 4 SpD Cresselia: 291-343 (65.54 - 77.25%) -- guaranteed 2HKO
So we are seeing some guarenteed 2HKOs from some of the heavier hitters of UU and OU. Choice Band Scizor with STAB + Technician boosted Bug Bite = 2HKO. That says it all for physical bulk.
Gengar LO + STAB Shadow Ball = Guarenteed 2HKO and that's with 252 HP and 4 SDef.
Going to try out some crit calcs now, I think we'll be seeing some OHKOs by Scizor and Gengar this time.
252+ Atk Choice Band Technician Scizor Bug Bite vs. 252 HP / 252+ Def Cresselia on a critical hit: 620-732 (139.63 - 164.86%) -- guaranteed OHKO
252+ Atk Conkeldurr Payback (100 BP) vs. 252 HP / 252+ Def Cresselia on a critical hit: 322-380 (72.52 - 85.58%) -- guaranteed 2HKO
252 Atk Life Orb Mamoswine Icicle Crash vs. 252 HP / 252+ Def Cresselia on a critical hit: 231-273 (52.02 - 61.48%) -- 95.7% chance to 2HKO
252 Atk Choice Band Garchomp Outrage vs. 252 HP / 252+ Def Cresselia on a critical hit: 373-441 (84 - 99.32%) -- guaranteed 2HKO
252+ SpA Choice Specs Heatran Fire Blast vs. 252 HP / 4 SpD Cresselia in rain on a critical hit: 256-303 (57.65 - 68.24%) -- guaranteed 2HKO
252+ Atk Choice Band Tyranitar Crunch vs. 252 HP / 252+ Def Cresselia on a critical hit: 564-666 (127.02 - 150%) -- guaranteed OHKO
So the only times we are going to be getting a OHKO on Cress without hazard or weather support is with a critical hit. Even then only super effective attacks will be getting the full KO. Cresselia is a beast, anf while she can tank many hits there are many she can't. TTar and Scizor are biggest threats in OU, and as most TTar run Pursuit you want to be careful of them. Scizor also generally run Pursuit so here's a calc without the crit,
252+ Atk Choice Band Technician Scizor Pursuit vs. 252 HP / 252+ Def Cresselia: 138-164 (31.08 - 36.93%) -- guaranteed 4HKO
and with a crit
252+ Atk Choice Band Technician Scizor Pursuit vs. 252 HP / 252+ Def Cresselia on a critical hit: 278-328 (62.61 - 73.87%) -- guaranteed 2HKO
Good luck killing that sucker.
Calculator link