So after getting the info from the AI thread and toning down the DPS on my CM team, I mastered it. The kill was messy because that bastard likes to chain stunlock my healer (and I even used double healers...) but it was still a mastery kill even with two deaths (2 damage taken medals lost, 1 for deaths).
This event was weird. The first boss took me two tries to master, because first time I killed him way too quick and didn't even realize my Edward was perma stopped the whole encounter and never got a chance to break him for medals. I had brought Ashe with her lightning BSB, Onion with Ashe BSB as RW, and Vincent with Ashe BSB as RW. So yeah, spider died in a few rounds, but Edward with his like 80 mind never came out of the spiderweb stop. I kicked off Onion for a second healer to lower dps and swapped the RW to OK BSB, which took care of mastery for the spider.
Then the second boss took me multiple tries, then an entire team re-work. Fun fact: Vincent with full magic synergy gear, Devotion RM, Memento Mori, using Maria's BSB as a RW, with the stacking mag boost on command 2 brings him to the hard cap on magic stat, which makes Maria's command 1 deal 9999x6 damage even without a weakness. Though like I mentioned, bringing that level of pain just gets you killed because you eat all his life % triggered scorches in a row, which is unsurvivable. Once again, I kicked off a dps for a second healer, and swapped RW to OK BSB, and that worked for the CM too.
Final U++ boss was a cakewalk one shot. Not even much to say really, he felt extremely out of place and should have been the first boss of the event. The spider with his pain in the ass stop + massive ultima nuke thing was a lot more threatening. Makes perfect sense, right?
My kills:
Spider Ultimate
Cid Mission Clone Ultimate+
Bahamut Ultimate++