Someone should also check how much standard Cresselia takes from Shadow Punch and see if her Psychic will be a threat. At a glance it looks like a possibility.
Damage: 122 - 144
Damage: 31.36% - 37.02%
Same Mummy against a 252/108 Cressy
Damage: 136 - 160
Damage: 30.63% - 36.04%
Modest, 224 SpA'd Cressy using Psychic on a 252/136 Hp/SpD mummy
Damage: 166 - 195
Damage: 54.61% - 64.14%
Timid 252 SpA'd Cressy on same mummy
Damage: 156 - 183
Damage: 51.32% - 60.20%
Slight problem with the Cressy vs Mummy numbers, there's no pokemon already in existant that have 90 HP and 110 SpD that is weak to Psychic attacks so they're off by alot.