Derek C. Simmons (rooftop)

Chapter: L5 · The Port, the High-Rises and the Quad Tower
Where: 5-5 Quad Tower Roof: a rooftop corner surrounded by zombies

Weak point

His body; he barely moves

A humanoid form weighed down by a mass on its back, almost stationary, with a scorpion-like tail of huge reach. The tail has a fixed rhythm and can't aim well past the corner: hide there. He impales zombies and absorbs them to heal, so clear the zombies first.

Reward

Access to the rest of the roof: 2 crates + 1 wooden box on the far side. Don't take the stairs yet: behind or under them, through the bars, is Serpent Emblem #20.

One source says the knife “locks” him if you get behind him; it's an unconfirmed rumour.

Step by step

Get behind the corner.
Kill the nearby zombies so he can't heal.
Peek out between tail strikes and shoot his body: shotgun, a few magnum rounds or a Remote Bomb.
It ends in a cutscene: the zombies swarm him.