The problem with a Rex is that we don't know how it's nervous system works, it may have a small brain, but still be able to function without it. It may be able to still kill you without being able to "think", what if you take out it's brain, and nothing really happens? What if its nervous system is spread out, and doesn't have one single point of failure like ours?
Biggest F'n gun I can find, probably something like those huge Russian antitank rifles, or a 105 Howitzer if possible. Probably try to bust up its hips or legs in general so it couldn't move around to easily, then retreat and let the big bastard bleed out.
If it had to be a "clean kill" Nothing less than a .50BMG weapon with any kind of explosive, or armor piercing rounds. Right for the presumed vital area, or Spine. I want it dead, and 5 minutes ago.
Edit: Woot 101!