Ok so I'm going to tell you by far the best way to solo take a Rex and his works for gigas too. Get a grapple hook find a Rex and lead it near a cliff. Grapple up the side of the cliff so you're high enough that the Rex can't bite you. Shoot it with 70 tranqs if it's a 150 and there you go you just tammed a Rex!(sometimes when the Rex's torpidity is high it will run and not come back to you so if it appears that the Rex is running from torpidity go to the ground and chase it and keep shooting it. The Rex won't bite you if it is running away) Hope this helped you guys out

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