Before you can go onto your next adventure, the final “raid” boss in Borderlands 2 is Haderax the Invincible. This creature appears after you beat Commander Lilith & The Fight for Sanctuary as an optional quest. However, with so many quests and things to do, it can be hard to locate, but it’s actually a lot easier than you might think.
After finishing the story, simply find Sir Hammerlock and accept his quest. There isn’t much to it, simply go to the creatures lair, give up some materials and defeat a giant worm.
To make it go faster, I suggest playing on normal, though it’s ultimately up to you. This fight is not as mechanically driven as some of the others and is really a last hurrah than an actual challenge. Simply killing the boss is enough to unlock the trophy/achievement.