G-Man Trophy in Little Big Workshop

Compared to the other clients, IncoInc's progression towards level 5 will take the longest. They offer little XP as rewards for their orders, though the pay can be somewhat decent. You'll need to reach level 5 with them before their final challenge will become available. The challenge will be marked with a gold star on it so you'll be able to clearly identify which one it is. Complete the challenge the trophy will unlock.

If you fail the final challenge, it will disappear for a few days. Completing a different IncoInc order should bring it back onto the challenge list in the Market menu.