Derek C. Simmons (dinosaur)

Chapter: L5 · The Port, the High-Rises and the Quad Tower
Chapter: A5 · Lanshiang by Helicopter and the Quad Tower: the Dinosaur, the Rooftops and the Walkway
Where: 5-3 Quad Tower Entrance (Leon) / 5-2 (Ada, from the helicopter): the plaza with the central obelisk, red barrels, a fuel tanker and a Humvee. **Story Intersection** with Ada.

Weak point

The eyeball inside his mouth, and the flesh on his back once you shoot the scales off. From the air (Ada): the eye on his back.

Pure brute strength: stomps, bites and a roar that disorients up close. He throws cars: instant death, so don't hide behind small cars. Use the central obelisk as cover. It's a long, phased fight, and between phases he reverts to human for the finisher loop.

Reward

On to 5-4 Quad Tower (Leon) or 5-3 Quad Tower Roof (Ada).

Offline, the Leon and Helena bots rarely start the finisher in Ada's campaign: in human form, your guns are the reliable damage. Every missed missile counts against your accuracy. On higher difficulties you have to mash faster for the finisher.

Step by step

First, loot the ring of crates along the walls (about 20), a First Aid Spray in the open area and at least one Remote Bomb in a box by the obelisk.
Lure him next to the red barrels and shoot them. Remote Bombs work too.
When he reverts to human: ram him, shoot his head until he kneels and do the finisher.
Humvee phase: a BSAA soldier arrives in a Humvee with 2 machine guns (infinite ammo, but they overheat: fire in bursts). Aim at the eye in his mouth when he lunges. It ends when he wrecks the Humvee.
Get him to charge the fuel tanker and shoot the spilled fuel to set him on fire.
Final phase: fewer barrels left. Remote Bombs by the obelisk. The magnum works, but save some .50 AE for the fly.
Ada (helicopter): circle him constantly while firing, and put the obelisk between you when he lifts a car. Break the plates on his back and shoot the eye there. Save the missiles (3-4) for his human form, and shoot the red barrels near him too. If he catches Leon in his mouth, hit him with a missile so he lets go: according to two forum threads, that earns the medal Rescued Leon from becoming B.O.W. chow.
Co-op in Ada Chapter 5: Agent sits in the co-pilot seat with his own gatling gun, and the missiles are shared. Let Agent handle the gun while Ada flies and fires the missiles.