Tiny Tina's Wonderlands

This achievement requires you to complete the "Best Chums, Bested" 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. I was able to do so without any risk of even losing my shield. Shoot columns to align them (you can't over-rotate them, so just shoot them until they stop rotating), or turn valves, and then as soon as the objective is complete, make a beeline to the portal and hop in to go to the next chamber. The only chamber that takes some time is the fifth, where you have to actually wait a seeming eternity for the water level to rise enough that the valves become active. The boss fight is a simple larger version of the shark creatures from the base game. Unload on it to finish it off quite quickly.

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.