With the influx of people from the Handsom Collection, I would just like to give everyone who has never fought him before some tips. These tips are assuming you want to have a “fair” fight against him as there are “cheap” ways to do so.
You can not damage him initially. Not until his shield is broken the first time.
How do you break his shield? With acid pools!
How do you make acid pools? Kill his worms.
HOWEVER, you must lead him to the center of these acid pools (you’ll know its the center from a little geyser bubble) for him to absorb it. If you don’t, the acid pool will consume the arena and you will take constant damage. You can use singularity grenades to pull him to a pool.
Master Gee moves in a fairly predictable pattern. For a large portion of the fight, he will be chasing someone. This person is in charge of leading him to the pools. It is best to approach pools straight on, then jumping over the center and continue moving in a straight line. If he follows, he will absorb the pool. The other people are responsible for killing the worms and, should one of you go in to FFYL, reviving them. It is not wise for the person being chased to do this unless they can phase rez.
Try to only have 1 (or no) acid pools in the arena. More pools means more damage and fewer places to avoid said damage.
Once the acid eats through his shield, it will regenerate. NOW your guns will deal damage.
BUT, worms will still spawn and create acid pools upon dying. The basic strategy does not change. Whoever is being chase should still lure him over the acid pools. It’s basically free damage per second if you utilize it, and a bane if do not.
In his final phase, who will lay down a purple bubble and anyone inside will gain massive health regeneration. This will mostly work to your advantage but there is a drawback. If all 4 of you survive and are in the bubble, it is very easy to be picked off by the rocket launcher if you are not careful.