i killed both the plesioth and green plesioth using full hermitaur armour and devil slicer. they are really easy to kill.
make sure you come prepared though. Do not even think about going on these quests before obtaining/making the following: sonic bombs - its hard to get him out of the water without these.
shock trap: - brilliant for getting a few good hits in especially where there are breakables on him e.g. fin. dont forget to make one before you go on the quest and make sure you take 2 genprey fangs and two trap tools to make more as they also keep him out of the water for longer.
bombs are great too but dont get too tied up in using them because they can get you killed if he attacks when your setting them up. your sword is the weapon you need really. bombs do tend to just get in the way unless you use them when hes in a shock trap.
no need to bother with paintballs or psychoserums really as he only goes in areas with water. also take lightning rods as these are effective against plesioth.
other than that, just get right in underneath him and hack and slash your way to victory. dont worry about him doing that move where he moves to the side as long as you have mega potions and potoins and herbs you will be fine.
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