I hated fighting Plessy, but I finally got to solo-ing him. I went with getting Felyne Watercaller to reduce damage from water attacks, as well as Defense (L) and Health +50 from food bonuses.
I was using a Long Sword to get him. I felt the Thunder LS in the game were pretty weak at that point, so I went with a Dios LS.
Here's a video I found for G-Rank Plesioth (not Green), only difference between regular and Green Plessy is the water beam they shoot while underwater - Green Plessy prefers to use the left to right sweeping beam, instead of the single direction one. Green Plessy also goes to land more often when enraged. Another tip I can offer is that you can use Frogs to pull him out of the water, he'll flop around on land for a long while when fished out.
Outside of that, I can't really help you with Ples as I hate killing Ples, and have only ever done it primarily for key quests to unlock other quests.
You can get a charmer set at 6*. Build a full Arzuros X set, find a Charm that has Charmer +4 minimum, and you're good to go - the set gives you +6. You can also add a bunch of other farming related skills - my Arzuros X set has Divine Whim, Speed Gatherer, Honey Gatherer, Charm Chaser, Anti-Theft, and Gourmand, I've only had to gem in Anti-Theft and Divine Whim, and I got Charm Chaser from a Charmer +4 charm.