plesoith

There's no best armour combination for it .. Just make sure it has water resist .. Just make sure you're equipped with Sonic Bombs ( screamer and gunpowder ) to in order to flush them out of water .. Next , watch where they exit the water , if you'd like trap them , leave a shock or pitfall trap in line with the path which they took when going in and out of the water .. I'd recommend you using a bow against plesioth .. So maybe sonic bow ? Also , you need to stand near it when using a bow .. Someone to the front and side of it's head cause it's water beam is real easy to dodge .. Funny how you're asking all the questions no one's answering huh ? Hahaha .. Remember to reply to this thread if you need more help ..

Well I defeated Plesioth with full Hermitaur for the first time but that's entirely up to you. Armor isn't a big factor when you fight him because the only attack that does a large amount of damage is easy to dodge, his water beam. For a weapon I suggest you use the Eager Cleaver. It's one of the easiest thunder attribute weapons to get around the time you fight Plesioth. I can't remember the materials but the iron katana weapon tree is as follows.
Iron KatanaYou can settle with the Eager Cleaver, or even the Iron Katana "Gospel" is your up for it.

If you prefer ranging then I suggest the sonic bow. It isn't too hard to make and goes as follows. GoFindItInTheStore
Easy huh? Good luck! :D