I actually did figure out the first part. I still had trouble with my attacks carrying me out in front of him and getting my ass kicked. Still, this is why I'm quitting the demo. I can no longer really have funny beating on Logombi and I don't want the frustration of this Plesioth fight to taint my experience of the full game.
Thanks for the second tip, though. That's something I may have not caught and should be really helpful for the full game.
Fair enough. No need to push yourself with something you're not enjoying.
Another tip that's relevant to both Plesioth and the full game. Monsters have limited stamina. It goes down when they do their more powerful attacks. It also goes down if you hit them in the body with a blunt weapon (Hammer / Hunting Horn) or attack (GS slap / SnS Shield bash).
You can tell their stamina is low because they start to drool. This slows them down, can make them trip, and adds lag to the end of their attacks. So, go on the offensive while you can. Plesioth is vulnerable to this in the demo.
A monster can restore their stamina in several ways, usually by eating or roaring.