STORY RELATED, CAN NOT BE MISSED!
This will come at the end of level 10 after you defeat Bertram and his T-Rex. There are three phases to this boss fight.
Phase 1 - T-Rex Lasers: Run into the parking garage as Stewie. This will keep you safe from the T-Rexs laser attacks and you will also be able to control the small Bertram enemies. As Stewie switch to your satchels and throw them at the T-Rex from the second floor of the parking garage. This will cause the most damage against the T-Rex. Also use the magma gun to defeat the small Bertram enemies. When you start to run low on satchels and ammo, keep killing the small enemies until they drop an ammo. Buying two health packs is recommended to stay alive. This first phase of this fight is the hardest. If you die, the T-Rex will return to full heath.
Phase 2 - T-Rex Rockets: This phase is a lot much easier. As Brian use either the sniper rifle or the rail gun to shoot the rockets on the side of the T-Rex. The rockets are heat seeking. I suggest staying close to the start of the second area as there will also be some small enemies running around. They aren't much of a threat this time around, but it helps to kill them when they are close by. If you run low on ammo you can run into the garage to the left and use the ammo cache.
Phase 3 - T-Rex Car Toss: Another simple phase. The T-Rex will now pick up cars with its mouth. Once the T-Rex has a car in its mouth shoot it so it blows up. Rinse and repeat until you defeat the T-Rex. You can use either Stewie or Brian. I used Brian with the shotgun.