The Forest quest is a fetch-then-fight loop: bring the woman by the lake three leeches, use the Leech Bait she gives back to hook the Giant Piranha, and return its skeleton to her for the flash drive that points to Island 3. The fight itself is decided in its second half, when the boss starts spawning small piranhas.
Find the three leeches
Talk to the woman in the straw hat near the shop; her counter shows 0/3. The leeches are small pale pickups lying on the grass in the middle of the island, not in the water and not in the swamp. Look down while you walk, because they are easy to step over. When you are close enough, a "Leech" label with an E prompt appears.
Carry each leech back and throw it at her with Q; she eats it and the counter goes up. You need three, not five. After the third one she hands you the Leech Bait (some guides call it the Modified Leech), which is the only thing that summons this boss.
Spend your money in this order
- Fishing Rod ($3): the long rod. The Leech Bait does not go on the Crab-Fishing Rod. Switch the bait slot with B once the bait is in your inventory.
- Shotgun ($150): the boss weapon. It hits hardest at close range, and the Forest shop sells the first bullet-damage upgrades for it. It has no Extended Mag slot.
- Pistol ($50): optional, but useful for the small piranhas later. A Laser Sight makes hip fire easier on both guns.
- Dynamite ($25 a stick): a few sticks for the swarm phase.
Cash for all of this comes fastest from the water. Beginner Lures cost $3 and bring up Cod, Pike, Carp and other cheap catches that double as food. Throwing a lit Dynamite stick into the water kills a whole group at once, but creatures killed by explosions never come out as Drip variants, so skip that method if you are filling the Fishipedia.
Two purchases pay off later rather than for this fight: an extra inventory slot so you can carry more catches per trip, and the $230 boat motor, which the later islands are much more comfortable with.
Before you cast, reload every gun and keep several dead fish in your inventory to eat.
Phase one: stay on dry ground
Equip the Leech Bait on the Fishing Rod, cast from the shore and keep reeling; if you only wait, the bite takes a long time. When the Giant Piranha comes up, fling it away from you so you get a few free seconds, then back off onto the grass.
The boss launches itself out of the water at you, and it can jump again every time it lands back in the water. Do not fight standing in the shallows or right at the edge. Move in circles around the trees rather than running away in a straight line, so its leaps hit trunks instead of you, and keep the Shotgun on it at close range between leaps.
Phase two: the small piranhas
At about half health the Giant Piranha starts summoning small piranhas. They are what actually kills most players: each has only 5 health on Normal but bites for 15, and they come in groups.
Thin them out before they surround you. A Pistol or Brass Knuckles kills one per shot or punch, and Dynamite clears a whole line: light a stick, wait for them to run at you in a row, throw it in front of them and step back. Keep returning fire on the big fish whenever the swarm is thin.
When your health drops, pick up a dead small piranha and eat it. They are free healing that keeps falling throughout the fight.
The Giant Piranha has 2,500 health on Normal (75% on Easy, 125% on Hard), and the white escape bar under its health bar still counts down. Patch 1.0.12 rebalanced this fight, so older videos may show it dying faster.
Hand in the skeleton
After the boss dies, the small piranhas it already spawned are still alive. Clear them before you search the ground. Among the meat chunks ($25 each, one per player plus two) lies the Giant Piranha Skeleton. Only the skeleton counts.
Throw the skeleton to the woman. She holds out a flash drive; press E on "Take Flash Drive with coordinates to island 3". Hold the Radar and a new dot appears to the north: that is the Desert island.
Optional: practise on a mini-boss first
If the Giant Piranha keeps winning, buy a Beginner Boss Lure ($40) from the same shop. It summons either The Old Pike or the Sunfish, which are optional fights with a much simpler pattern, and they let you practise reading a boss and its escape timer without losing your Leech Bait.
Next: Island 3, the Desert, where the grill and the Pufferfish are.
Source video