Well, that obviously depends on a lot of things. Specifically EV Spreads and what move is used. Ice Beam and Psychic are really the only moves I see though, so I will just use those. As Cresselia never runs offense, then I will just assume 0 SpA.
In that case:
+3 0 SpA Cresselia Ice Beam vs Calm 252 HP / 252 SpD Cofagrigus = 29.4% - 34.7%
+3 0 SpA Cresselia Psychic vs Calm 252 HP / 252 SpD Cofagrigus = 42.2% - 49.7%
or against the physical Cofag:
+3 0 SpA Cresselia Ice Beam vs Bold 252 HP / 0 SpD Cofagrigus = 40.6% - 47.8%
+3 0 SpA Cresselia Psychic vs Bold 252 HP / 0 SpD Cofagrigus = 57.5% - 67.8%