Master Gee The Invincible
Master Gee The Invincible is a raid boss exclusive to Captain Scarlett and Her Pirate's Booty. He, along with Hyperius the Invincible, can only be found upon completing the DLC's main quest line.
After completing the quest Hyperious the Invincible, Shade will have a new quest for you in Oasis called Master Gee the Invincible. To access the fight you must go to Hayter's Folly. Proceed through the area as normal until you come across a cage door with a lever beside it (Located at the far end of the crystalisk territory). You must pay 8 pieces of Eridium to pull the lever. Doing so will open the room allowing the true fight to begin.
When you first enter Master Gee's, like Hyperious, he will not be aggressive, to begin the battle you must attack him. be sure to wait until all of your party is inside the gate or they will be locked out for the duration of the fight. It is recommended, that if you wish to defeat Master Gee without using the safe spot (Explained below), then you should take a full party with preferably one siren to heal and revive players from a distance. When the fight begins there should be one or two worms in the room with Master Gee. Your goal through most of the fight is to kill the worms. Each time a worm dies it will create a small vent from the floor which releases a gas which will harm players. Once you have one of these vents, lure Master Gee into the center of it,you will need to keep him there until he has absorbed it all. The best tactic for doing this is using an assassins deception on top of the vent. Sometimes players will have to pick up the gas themselves in order to keep it from spreading and dealing damage to everyone in the party. If vents are left too long they will spread the entire of the area and all players will slowly take damage until they die. After roughly 5-6 vents of acid Master Gee's shield should be fully depleted, at this point during the fight your attacks will be able to cause miniscule amounts of damage to him, so shooting him is still fairly pointless, but for the most part the strategy remains kill the worms, one at a time, and lure him through the vents. His health should drop at a respectable rate from the gas, and if you manage to stay alive, he will die. Dropping between 3-10 Seraph Crystals.
 Safe Spot
The safe spot is an area where a player can sit and not be hit by the enemies in that specific area. Much like the safe spots that were possible whilst fighting Crawmerax the Invincible in Borderlands, also much like the safe spot I have wrote about on the Terramorphous the Invincible page. The idea with using this safe spot isn't the same as most others. In other cases you use the safe spot to attack whilst unable to take damage. In this case you use the safe spot, you sit, and you wait, and wait, and wait... and wait some more!
Using the safe spot is a simple and effective way to get past this Invincible for anybody who is having hard time and getting a little frustrated. Simply follow the guide above until Master Gee's shield is depleted (not necessary but highly advised as the shield will add roughly 10 minutes to the wait time). Once his shield is fully depleted run up to the gate where you first entered the room and to the left (facing the direction you would have entered) you will notice a rock just in front of the gate. crouch down behind here and move as far into the edge, between the rock and the wall, as possible. From here Master Gee will simply wonder around the area while slowly losing health from the gas he has already absorbed. The Sand Worm enemies will either stay underground or come in close and spit acid at you which will simply hit the rock. After the long wait, which I explained above, Master Gees health will be fully depleted however he WILL NOT die. Don't panic, it isn't glitched or broke he just doesn't die instantly. After around 30 seconds he will suddenly die and you are free to run in and fetch any loot he may have dropped.