Not sure if there is an existing guide for Plesioth Boat Fight but from how I see it, many are clueless so i thought it would be good to hurry and put up a guide. The guide is based on my own experience, if you have better strategy or explanation, please do.
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1. Get into the boat through standard interaction key.
2. Navigation follows only "W,A,S,D" and mouse is purely for aiming of bullets. Move mouse up or down to control trajectory and left right for pivot aim.
3. Steer well with good speed boost else you gonna capsize lots :D
Legend:
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Blue flags: gives ammunition.
Red flags: increases attack.
Mini-map:
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Spanners: shown on mini-map, is for recovery (boat repair)
Speed water channel: 3 super water streams available for fast movement because it gives unlimited stamina for short duration.
Utilize speed channels (hitting every cone) to move around during phase change to save on time and stamina!
You will have normal-1 and sky-shot-1 at the start of hunt.
Important note: Red flags seem to stack, ain't do much testing but it looks like it sure does.
The entire hunt can be identified as 3 phases.
KEYNOTE:
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Before and after each phase change (after Plesioth taking certain amount of damage), he will do this giant wave whirlpool and it hurts like hell while feeling helpless on the tiny boat so make sure to move away before it happens.
Phase 1: 0:46
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Gather up normal-2 from the blue flags around and get attack buffs from red flags. Utilize the rock cliffs to bait Plesioth charging into it for stuns.
Ammunition available: Normal2, Skyshot, Skyshot2
Phase 2: 4:07
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Once Plesioth reaches, it will perform the giant waves whirlpool follow by the summoning of Plessy-babies. Ignore the babies for now and damage until you see the flags arrive.
Ammunition available: Normal2, Scattershot2 (GREEN), Skyshot2
Phase 3: 7:31
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Congratulations reaching this phase! Same old giant wave whirlpool and plessy-babies summoning, but here, use the Scatter-2 shots from earlier to clear the babies (they will be a nuisance here definitely).
Fire away! When flags arrive, search for pierce3! (RED BULLET, they are red for a reason ;D )
The pierce3 bullets will hit like rockets but with high CD so aim carefully.
Ammunition available: Pierce3 (RED), CragShot (BLUE), Normal2
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Have fun hunting Plesioth! Hope this guide is able to assist with the community :D