This was our first time killing the Tier 3 Boss, the Celestial Indominus Rex Emperor. I think that our strategy worked quite well, we summoned him at the bottom of the waterfall and his AI caused him to basically get stuck down there while we pelted him with Demonic Mesopithecus "poop-balls". Cevk and Scrump definitely did most of the damage here, as the Celestial Wyvern had a hard time lining up a shot, and didn't do much damage even when it did. In the end we got the kill and a nice Celestial Helmet drop, which was very cool.