It is important to note that this requires you to complete 8 DIFFERENT levels in co-op. Luckily, you can play both controllers (offline only), and you can set the game to easy. Even on easy, though, it can be tough if you aren't watching both characters on the harder levels as one character dying means failure! To avoid this, just switch between controllers when one person is about to die and kill some enemies with them to get some health and Chi. Overall this is a pretty simple achievement and should only take about 2 minutes per level (so 16-17 total). It's nice to note that gold you get in co-op carries over to the main campaign, so perhaps doing this after you clear the first 8 lvls solo might be beneficial for you.