just complete the main co-op levels and unlock and complete the missing ones which can be found by finding the secret exits on the levels which blink and have a ? behind them on the world map.
There's loads of Youtube videos for most of the secret exits except for the one on 3-15. Luckily I found it and here's the video for it if anyone's still after it.
Credit to Bald Bouncer for recording and uploading this