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.
19 rexes health all about 25k dmg 550+ min saddle 86 highest 115, 1 yutti with good enough stats,should have worked fine for the alpha but the yutti ran forwards and attacked first before we could mount and died from the start ,recommend keeping the yutti on passive to avoid a disaster like that (alpha difficulty)