So I got one of these bad boys pretty early on using this method: scout out a beach or clearing with a Rex and lure away or kill any trikes, stegos, or other big dinos in the area, as I’ve lost Rexes who have attacked too many dinos and been killed (don’t kill everything in the area, as you’ll see in a second). Bring at least 60 tranq arrows (you’re going to need most of them) then lure the Rex into attacking another Dino it can defeat like a carbonymems or diplodocus. The Rex will be preoccupied with the Dino and won’t aground you so you can pepper it with tranq arrows without worry. Try to aim for the head as you get more torpor per headshot. Rinse and repeat until the big guy drops. Torpor drains really slowly so you can tame with regular raw meat and patience if you have plenty of biotoxin.