Using an Assassin and a Siren you can easily kill him within 10-30 minutes. First off, the Siren should hide in a corner where there is a boulder to the left when you first enter the arena to fight Master Gee. Once the Siren hides, you run around with the Assassin killing one sandworm at a time and the Sirincan sit back and help you kill the Sandworm you're attacking. Once you kill a sandworm a geyser of poison juts out. Use Deception on it as Master Gee is following you. He should absorb this poisonous geyser. If you get down have the Siren insta revive you. Keep repeating this process until he is dead.