They tens to spawn around the underwater rock formations, using an Icthy (preferably Megalodon as it can tank a few hits and a high lvl can kill off Plesis you don’t want) you can get the Plesi stuck in narrow crevices at which point you can easily fire away at it as it pokes its head through the gap trying desperately to eat you.

Be warned Plesis will sometimes fake fleeing and will run away temporarily before turning back around and attacking just as you get close, much like an Angler.