WROOOOM! Trophy in Little Big Workshop

When you get your reputation with Mitzurella up to level 5, he'll call you with a "final" challenge. Even if he doesn't call, this challenge will still be available through the challenge menu in the market and will be marked with a gold star on. If you have any other tasks running, stop/quit them or wait for them to finish before starting Mitzurella's challenge. You'll want it to have your full, undivided attention. Plan the task and complete and when you're done, the trophy will unlock. If you fail the final challenge, it will disappear for a few days. Completing a different Mitzurella order should bring it back onto the challenge list in the Market menu.

Mitzurella won't disappear after completing this challenge. He'll still be available in the challenge menu and through random calls for tasks. You can ignore these if you want but if you're in need of some cash, they should be easy enough to complete for a pretty decent payout.