I went in with two Daeodons eight megatheriums a yutyrannus and one megalania to kill the alpha version. I climbed to the ceiling of the cave with the megalania and blasted it with rockets from the roof while the megatherium tanked it and the yuty gave the Megas a buff and the daeodon healed
When you take on the brood. Rush it. If you get your rexs to surround it you can pin it so it doesnt do its poison attack. Rexs will need 30k hp 800 melle and you will need 10. For Alpha i suggest 85+ saddles 40k hp rexs.
Megatheruim are just as good as rexes in this fight so bring an army of one or the other but the sloth bois will give you a TON of chitin after the battle which is great for making ascendant chitin gear and saddles.