Tiny Tina's Wonderlands

This achievement requires you to complete the "Pesto Chango" quest at the highest (i.e. fourth) difficulty level. Each difficulty level works the same way: most rooms have you complete simple objectives, all of which are marked on your map. 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. Each run starts with some simple platforming where you need to head straight forward into a little castle, climb up and around the top and jump down a chute to get into town, kill a badass to get the ingredient list, and then hop in the nearby portal to start the DLC proper. These rooms are quite simple:
 

• In the first room, you need to collect five blue mushrooms. They're all marked on your map. It's very easy to just ignore all the mushrooms enemies and just run from map marker to marker to get all five mushrooms, then head to the portal and leave.
• The second rooms requires you to kill around five or six goblins in an area to get the badass to emerge, then kill it to take what it drops. You need to do this is three separate areas around the room, then jump in the portal and leave.
• The third room has you interacting with three separate ship helps to move a treasure chest to land, releasing a bunch of crabs. Kill them until one drops a crab leg item (you'll see the marker show up on your minimap when an enemy drops it), grab it, and move on to the next one. Pirates will endless spawn and swam you the entire time you move around this map, so just stay on the move, get each item and move right to the next, and then hop in the portal when you're done.
• The fourth room has five bone piles you need to interact with. Doing so spawns two regular skeleton enemies and one badass one. Kill the badass, take the item it drops, and run to the next pile. Do this with all five and then jump in the portal to leave, since there will be enemies everywhere by this point.
• The final room is the boss fight. Immediately head to the right, up the stairs, turn left towards the middle of the room and take the launch pad across the room to the other side. This landing area gives you a good vantage point to shoot Imelda. Drain her health, then she'll jump into the cauldron in the middle and respawn as one of a number of different creatures. Now, simply unload on her to finish her off and complete your run.

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.