After six tries, I finally 3* and commissioned Immortal Beast. Had to level my units up a bit after my third attempt, but it turned out pretty well. Good thing I got all the drops the first time, cuz I'm never doing that again. In the end, Sybilla and Bernice turned out to be my fodders. Bernice should have been my tanker for the Lich part, but she got killed by the Lichs' onslaught during my fifth try, so I decided to switch Clissa and Bernice's roles. Clissa made a good punching bag for those Lichs as she just kept on killing them while taking all those hits. The only other scary part were those orange giant orcs constantly killing Katie. It all depended on luck and my healers healing her fast enough before she got killed. Idk what is it with healers and their healing speed, but that screwed me over in the first few attempts.
This was my team going into IB:
As for DMM Aigis, I finished the first two missions of Sabrina's event. Guess I'll grind some more to finish the third one and maybe call it there since I'm taking it slow for my alt accounts.
Out of curiosity, how often does DMM and/or Nutaku use serial codes for Aigis? Nutaku Aigis seems to have the option, but I guess there's no serial codes for it atm.