In part one, use the Magnum. In part two, just shoot and wait for the rocket launcher.

Use the following strategy when fighting Tyrant on the helipad. You do not need the Magnum, as both the handgun and shotgun work equally well. Brad will always throw you the rocket launcher at about 1:38 on the timer, regardless of what weapon you are using. Keep tyrant in the same camera view as you. Get as far away as you can without changing the camera angle. Keep your distance and shoot with whatever you are carrying until Tyrant stands still and/or begins shuddering. Once Tyrant does this, stop firing and wait. Tyrant will charge at you. To avoid damage, run to Tyrant's right once he starts running. Tyrant's claw arm is the only arm he will use when he charges, and that is on his left side. If you run to Tyrants right (the side with his "normal" arm), you can avoid the attack, get your distance, and begin firing again. When the timer reaches about 1:38, Brad will throw you the rocket launcher. Quickly grab it and put some distance between yourself and Tyrant. The first rocket launcher shot will generally be deflected, and if he is close, Tyrant will slash you while you are lifting the weapon again. Fire the second shot to end the battle.