How do you catch the plesioth in the 5* old man missions?

Basically plesioth is weak against thunder. I recommend the SnS thundertip (hunter dagger -> thunderbane). I also wear protection piercing as well, the combination goes well with the SnS, as the plesioth can catch you out some times. Bring with you sonic bombs (gunpowder and screamer).
Barrel bombs are optional.

Use the sonic bombs when it hasn't been out of the water for a while. It will get it out of the water most times you use them. when the fins go down on the back of the plesioth, you know it is weak. That is when you have to place the pitfall. Wait until it is out of the water (obviously) and it shoots the water spray from it's mouth. Run underneath it, and place the pitfall then.

Attack it when it shoots the water spray, run either to it's head or to it's leg and hack at it there.

Watch out for when it rams it's body (annoying) It can easily catch you out. It is why I like to use protection piercing against it.

Also the swinging of tail is annoying. Not only does it reach far, it reaches further than it looks...

Don't attack it when it has just jumped out of the water, and is wriggling it's body across the ground.

And watch out for when it runs back to the water.

That is all I can think of...