Upgraded machete to cut the root wall (needs Deep Sea Ore)
Attack patterns
Neck whip โ long reach, can swing in front AND behind; fire a few shots then move to the far side.
Head slam โ rears back and slams down; swim toward the ceiling to avoid, then dump damage into the neck while it's stuck (best damage window).
Shell spin (low HP) โ retreats into shell and spins; move to room center, dodge up or down as it sweeps top then bottom.
Strategy
Pre-fight: talk to Basu and the village chief, then Sato, about the turtle.
Find the den on the left of the lake (~60m); swim straight to the roots to trigger a cutscene that forces off the patrolling Giant Sturgeon โ don't fight it.
Gather Deep Sea Ore and upgrade the machete, then slash through the root wall (hold B/Circle until Dave glows blue, release).
In the fight, focus the neck with the shotgun; punish every head slam, and weather the shell spin from the room's center.