There's only two real counters to cresselia: You will need to be well prepared with top counters, correct moveset, powered. Max combat power by level this chart shows the maximum combat power for shadow cresselia at each level.
This means shadow cresselia has perfect 15/15/15 individual values. Shadow cresselia is at its best in the ultra league, overshadowing regular cresselia with impressive new wins such as giratina altered, guzzlord, jellicent, and even beating regular. If the cresselia you encounter after you defeat the raid has a cp of 1633, it is guaranteed to have an individual value (iv) of 100%!
Find detailed charts for shadow cresselia max cp, cps from field research, eggs, gbl, and raids. Explore cp ranges, levels, and all possible iv combinations. Cresselia is tanky, so it is probably best to have at least a group. Check out what is individual value here!
So any weather boosted shadow pokémon will have a minimum value of 4 in each stat. The creature will be available in the wild area. Bite/crunch tyranitars shadow claw or lick/shadow ball gengars. The only current exception is whatever pokémon giovanni gives, which has a minimum.