Seeking advice for 8-Star Troubled Waters

I am currently stuck on the eight-star Village quest, Troubled Waters.

It is a quest requiring the player to hunt a Lagiacrus and a Plesioth on Deserted Island.

I currently wield / wear:

A full set of Barroth S Armor (gemmed to AuL) and I wield a Gun Hammer, though (884 ATK, slight blue sharpness, 100 Fire, Affinity 5%.)

I get triple-carted because Plessie and the Electric Eel keep ganging up on me.

Dung Bomb Lagiacrus. You can get him to change zones into dry land. Plessy never leaves the water zones, so you can decide which of the 2 you want to hunt first, then go for the other.

I thought you learnt your lesson in the last quest: don't fight both at the same time. Leave the area yourself if you have to but don't fight both at the same time.

I know about not fighting multiple large monsters simultaneously.

I just got so excited (I finished the seven-star quests, and was quite pleased with finally finishing The Write Stuff and Rumble in the Great Desert that I guess I pressed my luck too much.

I've got a new plan right now:

1) Battle the eight-star Agnaktor in Denizen of the Molten Deep roughly one quilliard times (I hate the desire sensor) until I get one more Agnaktor Claw+, two more Agnaktor Hide+'s, and one more Agnak Carapace. Then I can upgrade to a Flamestorm+.

2) I'll use that to rip Plesioth and Lagiacrus apart!

Actually, I've began using Switch Axes, and to quote Han Solo from Episode VII, "I like this thing!"

Any good Switch Axes and armor combinations that are available at eight stars?

I just re-fought the battle, and I wore a full set of Barroth S Armor, gemmed to Capture Guru.

I captured Lagi, but Plessy proved to me that Capcom chooses its opponents foolishly.