Easy tame.... Just make a 2x3 stone foundation and cover 3 high with doorframes on the sides. And 2 high on either end. Place wooden ramps (They're temporary but easy to make and repair) just get aggro and run inside the trap. Takes about 30-45 tranq arrows from a mastercraft longneck. On mobile the kibble is easy enough to make. Get a fast mount with knockback. Go to fars peak and run around pulmonoscorpions to find their eggs. The kibble needs prime jerky and longrass so if you have that set up, rush a few caves give it a soothing balm and under level 70 only needs one kibble. So that's a convenient benchmark. It's expensive to build the trap big enough to get the red out so once tamed just request it back to base. Unless a Giga or enemy tribe shows up it will be untouched if left alone.
The way I tame Rexes is to go out on a tapejara and a bow and tranq arrows. Cling onto the side of a cliff and non stop pump it with tranqs. Throw some raw meat in so it starts the taming process and quickly go and kill stego or something else to get raw prime.