Just completed alpha broodmother boss fight with single player settings disabled (and at default settings for everything except time related stuff like raising timer, taming, etc.) but still on a single player world so I don’t know if it’s nerfed or not compared to a server. I had one yuty and nineteen rexes all with 35-37k health and 1000.8% melee (15 mutation stacks for both hp and dmg to get those stats with good wild stats) with saddles ranging from 70-90 armor. It was a success with zero casualties to my surprise but some were very low while others were at half. Make sure to surround the broodmother to make sure that every rex is able to attack without being body blocked by other rexes. Would still 100% recommend megatheriums but rexes are still viable for this fight just like literally every other boss in the game short of moeder and the dessert titan. Good luck survivors!