Derek C. Simmons (fly)

Chapter: L5 · The Port, the High-Rises and the Quad Tower
Where: 5-5 Quad Tower Roof: the window-washer cart, then the helipad with the broken lightning rod

Weak point

The golden joints where the legs meet the body; the eyes once he collapses. Normal shots bounce off the shell.

Leon's final boss. First, from the window-washer cart, Leon shoots the fly's head and eyes. On the helipad comes the final form: the pincers cover the eyes, so break the leg joints. Each lost leg makes him absorb zombies to regenerate, and that's where the lightning rod comes in: impale a zombie with it and, when Simmons eats it, lightning strikes him. The magnum is the best weapon (save it all chapter); the shotgun and assault rifle also work.

Reward

End of Leon's campaign (achievement The Trouble with Women).

The helipad zombies never stop coming: killing them gives ammo. No source publishes his health.

Step by step

Split, the window-washer cart: Leon drops into the cart and shoots the fly's head and eyes and its spit globs; from there he can't aim towards Helena. Helena runs the cart switch while zombies swarm her: the faster she is, the sooner you regroup.
Helipad: loot first: 2 crates + wooden box on the far right, 2 crates + wooden box in the upper-left corner and 3 crates in the far-right corner.
Shoot the golden joints on the legs. About 2 broken legs is a good pace.
If he swipes close to you, a QTE appears: succeed and your character climbs the leg and plants a grenade in the joint.
Pick up the broken lightning rod and impale a zombie (prompt next to the zombie). Keep damaging him until he eats it: lightning, massive damage and a stun. Break 2-3 more joints while he's stunned.
The rod drops: pick it up again. Co-op: one player carries the rod and impales zombies; the other shoots the legs and keeps the zombies off them.
After about 3 strikes he collapses: stab the rod into his eye, or impale another zombie and shoot the eyes.
Last shot: by the helicopter, take the Rocket Launcher and fire at his eye as he climbs up.