Plesioth

Use a lgihting weapon and bring lots and lots of sonic bombs, too keep it outta the water use shock traps

If you are using a melee weapon:
Use a Thunder or Fire Weapon. Bring sonic bombs and throw them at it when it's in the water to get it out. Use traps to your advantage by setting them in places it goes to return to the water.

If you are using a long range weapon:
Use Thunder, Fire, Pierce, crag, clust and poison shots. When the Plesioth heads in the water shoot a crag or clust at it and it will come out again. Then poison it and use either of the three pierce, thunder, or fire shots and keep shooting. If you're using a bow bring sonic bombs and shoot at it when it comes out. Remember keep a far distance and get higher if you can.