Tiny Tina's Wonderlands

This achievement requires you to complete the "Outer Daemons" quest at the highest (i.e., fourth) difficulty level. Each difficulty level works the same way: most rooms have you complete simple objectives, but they unfortunately aren't all marked on your map in this DLC. If you're playing at level 40 with all level 40 gear, you can likely just ignore all enemies and run around completing the objectives. These rooms are a little more obscure with what to do compared to the previous DLCs, so I recommend actually reading through so you aren't running around getting shot trying to figure out what to do.
 

• In the first room, you need to collect five golden rocks. Grab the first one and then pull the nearby lever (marked as an objective) to send a mine cart into the wall (note: for me, the cart just sat by the wall here and I had to shoot it, but it was only the case with the first wall, not the others). Now you can grab two more rocks marked as objectives, then you'll likely need to follow the tracks to find the next lever to send another cart to a wall. Now you can grab the fourth rock, then kill the gold golem to get the fifth, allowing you to jump in the portal and leave.
• The second rooms tells you to kill enemies and then destroy their standards. Standards are flags. Look out for the long hanging red flags, and you'll find around ten enemies nearby. You must kill them all before you'll be allowed to shoot the flag to destroy it. Repeat for four more flags in the area, then leave through the portal.
• The third room requires you to find a object under one of the red Wyvern eggs. There are five eggs to my knowledge, and it can be under any one of them. Two red eggs are on the top level (take the launcher up from the start), one on the tower in the middle and one nearby on a ledge. Drop down to the second level of the tower in the middle to find a third egg. The fourth and fifth eggs are on opposite sides of the tower on the lowest level. Break each until you find the objective item beneath one, then leave through the portal.
• The fourth room requires you to kill a bunch of skeleton enemies until eventually a troll enemy will appear. Kill it to get a lever, then use it on one of the marked spots in the center of the map. Repeat this two more times to get two more trolls, two more levers, and use the last two spots in the center. This spawns the portal on the outer rim so you can leave.
• The fifth room tasks you with searching for hellfire. First, just kill enemies in the courtyard area, and once you defeat enough/all of them, the big castle door will open (and you'll likely see a yellow health bar appear for Raging Wyborg). Head inside the open door, up the stairs and ladder, platform up and outside the wall, and then take the purple launcher in front of you to land where the boss fight is. This boss has two phases: drain its health bar once, then you'll have to place a mine on it (green spot on its head). Then, it'll refill its health halfway, so simply drain it again.
• The final room is the boss fight. It's a dragon that flies around overhead, and surprisingly doesn't have any special phases or anything. Simply drain its yellow and then red health bars to finish it off and complete the DLC.

Now here is the critical part. Completing a run theoretically unlocks the next difficulty level, but Gearbox has made the decision to only introduce one new difficulty level per week. This means you would potentially have to wait a month to have access to all four difficulty levels. There is thankfully a way around this barrier: force close the game app, go into your settings, Network settings, and choose to 'Go Offline.' Now launch the game, head back to the mirror, and you should get an option to scroll right to increase the difficulty after interacting with the mirror. Do so, and complete it on difficulty levels 2 and 3. With that done, and now that you can access and see difficulty level 4, while still in the game, open your Network settings and choose to go back online. Now simply play through and complete the run on difficulty level 4, and this achievement will unlock as soon as you kill the boss.