Tamed one can defeat a rex.

Real, just try to not get rex near the trike, attack he and knockback, repeat

A plan:

[Rex] – "walks to trike'

[Trike] – "waits before rex CAN'T attack, but he can"

[Rex] – " Can't attack, but can be attacked"

[Trike] – "Attacks and knocking back rex"

Repeating process.