For 1v1? A 100% Cold Platinum Drake with Chiv+AI. Run Consume Damage on it with 120 Taming/Lore, and the T-Rex basically can't damage it. You just have to worry about the T-Rex paralyzing you and switching targets to you.
For taking it out in a group, a Frost Mite with RC can greatly increase the damage done to the T-Rex. A guildy of mine runs Death Ray as his pets apply RC and Conductive Blast to it, while i run Discord and use my 100% Energy Platinum Drake with Chiv+AI to kill it.