After playing a number of matches once the changes were implemented i can say that i was right in my suspicions. There are no really viable checks to it at present (don't give me some bullshit about pinsir being a counter). The fact that i am forced to run a poke - NPdoom -specifically designed to combat it is a testament to its influence on the current UU landscape. Even that is not sufficient as every team i've seen also carries a chansey for this very reason. Teams that I previously could rip through 4-0 now take me to 1-0 or worse, thanks to Dual Screen and lunar dance. Trust me, theres nothing better than to see a chansey on 8% restore all its hp and pp for softboiled.
You only need to look at how it fares against the most powerful physical and special pokemon to gauge just how broken it is.
Su(b)perior (Megahorn) v DS Cress: 45.5% - 53.6% with NO SCREENS UP
23% - 27% w/ screens
LO Alakazam (Signal Beam) v DS Cress: 38.3% - 45% with NO SCREENS UP
18.9% - 22.5% w/ screens
Is it broken defensively? yes
Is it broken as support? yes
Is it broken offensively? can be due to its high defenses, Screens and access to charge beam/Calm mind
The elusive triple threat suspect. If its not banned next time around, i'll be very, very surprised. Really, if it could fuck off tomorrow that'd be great.
I don't know what you are smoking, Cresselia is no where near as a big of a threat that you are exaggerating. Any Cress that doesn't carry Rest gets stopped completely by Toxic. Also, several Pokemon can easily get pass Cresselia.
Taunt SD Drapion can set up in Cress's face and after an SD, does 72.1% - 85.6% with Night Slash, always a 2HKO while Cresselia can't do anything back.
In addition, SD Absol destroys Cresselia. A +2 Adamant Night Slash from Absol does 93.2% - 109.9% to Cresselia, always a OHKO after Stealth Rock. Even behind a screen, it does 47.3% - 55.9%, still a 2HKO after SR and Leftovers recovery.
Cresselia has been strong, no doubt. But you are overly exaggerating it's abilities. In my experience, Cresselia has been incredible, but not overwhelming in any way. On my teams, I have been using a SubCM Cresselia and a Thunder Wave/Charge Beam/Ice Beam/Moonlight, both have been awesome sweepers. Cress is pretty much an amazing Defensive Pokemon, pretty much an upgraded Uxie and Slowbro, with the ability to be an amazing sweeper as well.
It will take time to see if Cress is broken or not, but so far, at least to me, it hasn't been too troubling but is definitely a threat.