Spoiler Alert *****
You will complete Act 3 as soon as you kill a boss named JackLeg Joe. This is a solo mission and cannot be completed as a party. You will keep doing the main quest line until they tell you to go kill him. You will end up fighting your way thru a tunnel killing off random enemies , nothing to tough until you enter a big room and a cut-scene will start introducing you to Joe. Do NOT attack Joe directly. He has a shield and cannot be hurt. Just stay back and pick a side , left or right and hide behind 1 of the 2 large barricades. Sit back and kill all of the waves of enemies that come out. There will be quite a few , nothing too tough tho. Eventually another cut-scene will start showing the NPC that follows you shooting a smelter enemy , causing his backpack to catch fire. He will run under Joe and explode causing his shield to empty for a brief moment. This is when you attack. His shields are only down for about 4-5 seconds so unload all you have on him. Once his shields are back up just stay where you are hidden behind the barricade. Most of the enemies will be pre-occupied with your NPC friend. When the smelters come out just wait for them to come to you. Melee them knocking them to the ground , then shoot them once in the back pack causing it to ignite. They will then run under Joe and explode giving you a chance to take some health away from him. Once you have Joe down to about 1/3 health all enemies will disappear and Joe will jump down. RUN as fast as you can in a big circle. He has a grenade launcher and fires tons of grenades. Do your best to just keep running and shoot him when you can. NOTE..... If You Die At Any Time During This Fight , You Will Be Re-spawned Outside The Room And Have To Start The Fight All Over From The Beginning. Here is a video I found after I killed him. It is not my video and i take no credit for it. all credit goes to the creator. Thank you and if i missed something please let me know.