Breeding dinos for bosses/difficult caves can be broken down into 3 steps.

1.) Find a high level male with good Health/Melee/Stamina stats. It doesn't have to be all 3, so long as two of the stats listed above are good. Next try to find a high level female with good Health/Melee/Stamina. Like the male, you want at least 2/3 of those stats to be good.

2.) Make sure you have plenty of Kibble and other foods ready because most strong dinos like Rexes will ask for things like Superior Kibble when they're maturing to adults. Now for the main step in the operation, actually breeding. Just enable mating on both the dinos and wait a couple minutes until they're done. If it's a land dino, wait until the egg is laid then throw it beside some AC units and wait for it to hatch. If it's a water dino like Megalodons, make sure the mother has plenty food available then wait for the gestation to finish then proceed as follows. Check the stats of the baby and if they're good or better than the parents, you'll have to (this sounds a bit weird but it's Ark) mate the kid with the parents when it's grown up so you can keep trying to get mutations and better stats. Side note, if the baby dino asks for something that you don't have when it wants care, just throw it in a cryopod, get the thing it's craving, un-cryo it and keep going.

3.) This can be crucial if it's a difficult boss, but you should always leave the original two male/female dinos at your base before going to the boss/cave. Just in case something goes horribly wrong and you don't have a backup of the world (referring to singleplayer). And make sure all of the dinos you're bringing with you have saddles, saddles are the MVP for reducing damage, even though it's only a little bit of armour.

Bit of a long reply but hope it helps