To beat Tyrant, I would recommend you using a hand-gun. His heart is exposed to the outside. So that should be your target. I got this information from a gaming website. (Source given below)

--quote--
When the Tyrant starts to walk toward the characters, aim at his outside heart
and begin to blast it. A handgun works just fine - don't even switch to
another weapon unless you're in a hurry.

---------------------------------- ITEM(S) ------------------------------------
Green Herb - While fighting the Prototype Tyrant look to the right to find
this on the floor, or when stepping behind his back look to the left. (NOT ON
HARD MODE)

The Tyrant will begin to stagger as his heart is blasted and this is when you
should reload. Don't wait until the handgun is completely out of ammo to
reload! He will hit both characters if he is not stunned every few seconds.
He will stagger after just about every 3 shots.

The characters will eventually move behind the Tyrant. Wait for him to turn
then keep laying down fire on his heart. He will fall shortly. While in the
back of him, you may want to switch to the Submachine Gun since he will attack
more often from his closer range. Stun him by shooting him in the heart like
before to keep him from attacking. Oh, how the less mighty can still have
their ass handed to them just like in REØ!

Equip a Submachine Gun unless you have some really good aiming because once the
elevator doors open, a group of FOUR ZOMBIES will attack. Shoot any of them
that try to take a swing at the characters in the hope of stunning it. The
characters will step into the elevator after the fight then take it down.
--/quote--
http://www.gamefaqs.com/console/wii/file/933020/51132

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