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)
My first fight with three friends we failed the second we were strategic get two people to draw its attention and then the other two should attack the broodmother and the spiders we did very quickly. Try it
My method for an early kill is to bring a few high level turtles and then have a Daeodon for health boost and some rexs for damage. Maybe a yuty if you manage to tame one but without one I’ve done fine