Because Cresselia is a pure Psychic-type Pokémon, your best bet to counter it during raids is going to be Dark-type, Bug-type, or Ghost-type Pokémon.
Pokémon Go has recently added your best friend for this Mega Beedrill. If you have enough Mega Energy, bring a Beedrill into the battle and use Bug-type moves.
Gengar is another common Pokémon that can be useful, but make sure that you’re using one with Ghost-type moves rather than Poison-type moves. Because Gengar is part Poison, it can suffer from a bit of a glass jaw when facing Psychic-types like Cresselia.
