Since rexs are the second best land carnivore (behind giga) I would recommend just leveling it for PvP. Like on all my 150 kibble tame rexs I go for 10k health then 500 melee damage and then put the rest of my levels back and forth between health and damage.
PS: A good 2nd-5th gen Rex can and will kill tamed gigas
When breeding two max level rexes to have a super baby, DON'T EAT THE EGG!!!! I've done this twice on accident(meaning to drop it) and each time it takes me a couple seconds to register wtf I just did and then I have to wait to mate again. Sad day. Weekend ruined. Great way to start off the new year! Lol
Rex baby consumes meat at about 1 meat per 50 seconds. Each meat gives about 50 food, with a food drain of 1 food point per second. So.. 1 meat = 50 seconds of feed. 1 stack of meat is 1000 seconds/16 min. A stack of meat takes 12hrs to full spoil. So.. on 1x, at 6% maturity, a baby rex can be left to fend its own till juvi... this is even with a 2 hr safeguard to be very safe. Honestly... 5% would work also. Hope these statistics help!