The Last Kids on Earth and the Staff of Doom

This achievement isn’t unlockable until after the main campaign is complete. The various characters around the town square will have new post-game quests that are both super easy and very lucrative, with each one offering a reward of up to 10,000 scrap. (So if you’ve reached this point of the game and still have upgrades to buy (and that’s likely), these quests will provide a significant boost).

Assuming you’ve kept up with the various sidequests offered at the town square throughout the main game, you should only need to do a few for Melvin and Globlet to pop this achievement.

Good luck!