Successfully complete 5 laps in an endurance game on Hard - 30G
If you have the DLC I suggest you attempt this on Sting of the Bumblebee, you can miss many beats and still keep the multiplier. There are stretches of Blue, Blue, Yellow, Yellow and you can totally ignore the Red & Green during these parts and still maintain the multiplier bar. Then it leads into Yellow, Green, Red and you can ignore the Blue and then it alternates to Green, Yellow, Red.
Once you get this pattern down you can easily make it to Lap 5 consistently. Then it is just a matter of not missing any beats because that is where you will lose the multiplier. If you try to hit all the beats you will surely fail, it's all about picking which beats are the easiest to land and go from there. I did this in about 1 & 1/2 hours and you need to get to ~ 9:30 to reach Lap 6.