Tips for bosses?

Playing on SP. I wanna chaenge the alpha bosses soon. What are the best creatures to go for mutations?

Ik Megatheriums are the best for Broodmother and Therizinos for the dragon. Might use the Theris for the Megapithecus and Overseer as well... Unless there are better options? How about Rexes or Spinos?

I also imagine you cannot bring Gigas or Carchas into boss fights/the Tek Cave.

Also I need advice on how to speed up mutations, cuz in SP time doesn't pass when you're offline so you can't just mass breed and stack up eggs due to the mating cooldown on female tames.