I play on PVE so I can't use high tech tactics. So far I've tamed about 6 Rexes, 3 of which were lvl 145. (Still looking for the elusive 150)

Anyway the tactic that seems to work the best for me is to kite the Rex to a large shift in terrain. Something the Rex can't walk over. Honestly doesn't have to be much tho, they seem to get stuck on even smaller rock formations.

Once its stuck hit him with a bunch of trans arrows. (You can always break the rocks with a pick axe to get it out once it's tamed.)

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