Microsoft Jigsaw

This will be a cakewalk. All you need to do is complete 10 puzzles on easy, which shouldn't be hard for even beginners, most easy puzzles are about 24 pieces, if not a little more. Will definitely unlock when going for "Investigator." 

Note: This will only unlock when the counter on the main menu says you have completed 10 puzzles, not when you instantly complete them. Sometimes this game only counts a puzzle as played instead of completed, so you need to keep playing until it counts 10 completed.(Credit goes to Kairi).