- Note: This mission appears only in Normal and Hard mode.
After completing the previous mission, you will see a message that the bombs could not be disarmed and you have to evacuate immediately. The game will not wait for you to press a button before continuing to this mission, so be ready.
First off, you don't have any time to mess around. You really must finish this mission as fast as possible. If you're lucky, you'll have ten seconds left, but it may very well come down to the last second.
First start by running right. You've been given 20 rounds for the heavy tranq bazooka, so switch to that while you run. After the music kicks in and the screen starts shaking, run back to the left. You will see the T-Rex. The general strategy here is to run under the T-Rex, point up at his head/neck, rapidly press 
a lot to fire tons of shots into him, then quickly run back to the right before he chomps you. Be sure that all your shots make contact, especially while using the heavy tranq bazooka, because you need all the help you can get to take him down before time runs out. When your heavy tranq bazooka runs out, switch to the normal tranq bazooka, then the shotgun (or the other way around, but it will switch to the normal tranq bazooka automatically anyway). If those run out, either the lethal machine gun or the tranq machine gun should do fine, but make every shot count!
Eventually, the T-Rex will force you to the far right. You will find two ammo pickups here, but they give less than the usual amount of ammo. For example, instead of 20 shotgun shells, they will give only 5. You can elect to pick up ammo for the heavy tranq bazooka, but they will give you only one round.
To avoid getting chomped while you're at the far right, you may want to jump out over the pit slightly when he finishes roaring, then jump back to the ledge in midair. It's easier than it sounds.
If you're playing on Normal, you've beaten the game! Otherwise... prepare to take down the Biosyn Helicopter.