Brzak

Chapter: L3 · The Catacombs
Where: 3-4 Underground Water Channel: the flooded channel, the iron gate and the rapids at the end of the chapter

Weak point

The glowing orange tongue in its mouth, which only drives it back; it dies to a scripted explosive barrel

A giant shark-like B.O.W. You don't fight it: you run. Don't waste ammo on its body (one source gives it 150,000 health in the first encounter). Every time it advances, shoot its tongue to drive it back; if it reaches you, it's instant death. The kill comes on its own at the end, in slow motion.

Reward

End of Leon's Chapter 3 (achievement Get on the Plane).

Any fast-firing weapon works on the tongue. Health from a single source: 150,000 in the first encounter and 32,700 in the tunnel, so it isn't meant to die to gunfire. Between the chase and the rapids, Helena's path has 5.56mm ammo and Serpent Emblem #12.

Step by step

First encounter: a series of QTEs, slightly different for Leon and Helena. Follow the on-screen buttons.
Split, the chase: Leon swims along the torches to a gate Helena has to raise. Mash the swim button to outrun it. If a corpse grabs him near the end, rotate the stick.
Helena, on the rope bridges, shoots the Brzak's orange tongue to save Leon. If either of you fails, it's game over. Open the gate together.
Rapids: a waterfall sweeps you along. Grab the 9mm Ammo that appears. Each time it advances, shoot the tongue repeatedly and reload in the gaps. Dodge its bite (a 2-button QTE) at least once.
Finish: explosive barrels roll down the tunnel and the game goes into slow motion: shoot the barrel above it when it's close.