Irving

Chapter: 3-3 · Drilling Facilities
Where: The patrol boat, on the water. It has **2 Mounted Machine Guns** and **2 Mounted Launchers** toward the bow, and a First Aid Spray at the stern.

Weak point

Phase 1: the bulging middle (the black mark) of each spiked tentacle and, when he surfaces, the pale circular patch on his "forehead" (flashes red when hit). Phase 2: Irving's human body on the tongue and the tentacles gripping the boat.

Irving injects a parasite derived from the control Plaga and turns into a giant fish-mollusc with tentacles and an oyster-like mouth full of teeth. He is fast in the water, but his rapid mutation leaves a core exposed.

IGN: use only the turrets and save your own ammo. Man the Mounted Machine Gun on the side he is on; your partner takes the other machine gun (if he is behind) or a Mounted Launcher (if he is at the side). A missed QTE often means instant death: keep your hands ready and step off the turret to dodge.

Reward

Nothing. His dying words point to "the cave". "Irving (Transformed)" figurine: 5 BSAA Emblems and Chapter 3 cleared.

HP by part (Famitsu guidebook via Fandom): parasite 12,000, tentacle A 8,500, tentacle B immortal, Irving's body 13,000. Damage: spike 200, whip 180, bite 300 or instant death.

If you run low, damaged tentacles can drop ammo. IGN calls it "laughably easy"; Fandom "extremely annoying" because of the QTEs.

Step by step

Phase 1: he submerges and raises spiked tentacles (IGN: four) on one side. They lock onto where you stood when the attack began: keep moving. Destroy them by shooting the bulging middle.
He surfaces: shoot the pale patch on his head. Then he charges and bites: dodge. Repeat on the other side.
Watch his map icon: if it charges the boat, the bite over the deck is coming (QTE or move away). If a tentacle grabs someone, the other player shoots it to make it let go.
Shortcut: one hit from your own Rocket Launcher (the 3-1 one, not the turret) anywhere on Irving skips all of phase 1.
Phase 2 (stern): cutscene; he grabs the stern and opens his mouth. Shoot Irving's body on the tongue and the tentacles holding the boat, using both machine guns.
If you don't do enough damage he lets go, submerges and lunges to bite again: back off or follow the QTE, and repeat.