Help!!

Use a bow. Stay far away from it when it's out of the water watching for the water shots. When it's in the water run torward the edge of the water staying a bit to the left or right side of it. This way the water it shoots either in the sweeping motion will be too high for it to hit you, and the water it shoots straight out won't get you. During these times just shoot. Theres a chance it might die in the water so bring some sonic bombs (Note: Sonic bombs do not work on creatures when enraged).

I suggest you use a bowgun with an attack of 250+ and use lv2 pierce and lv2 Clust against it

I would suggest using a Sonic Bow II or Sonic Bow III to kill it... With the 120-140 Thunder element and 180-192 attack with a long range, it's practically made for the plesioths... Stay out of range of it's shots and when I goes underwater to shoot at you, once again stay out of range but stay close and run from side to side (preferably with bow drawn so you don't stop and turn and about half a second in between each turn)... If it still looks at you while your doing this theres a 50% chance it will pop out of water... Only try doing that after you use all your sonic bombs... If you stay close to it when it's out for it too shoot you with water, but far enough away so you don't get hit with physical attacks, constantly shoot at it (dont charge, it takes longer) until it's about to charge up to shoot you then just move to the side to get out of range of the shot and repeat... The process may take a while (example:: my first plesioth with Sonic Bow I, killed it with 49 seconds left) so pace yourself, but keep pace.... One last thing, if you follow this and you're not an idiot getting hit by the water or being trampled, then it killing you shouldn't be a problem, you killing it in the time limit is the main problem... Good luck