This achievement can be done on any difficulty combination provided but obviously on some songs it will not be possible to reach 200k at lower difficulties.
With an Expert/Expert combination it is possible to do this on every single song.
For those of you playing on lower difficulties a couple of good songs to try this on are Jessica, Free Bird and X-Stream as they are fairly long songs with a decent number of notes.
You have no choice but to activate star power at the same time on GH2 however you do need to make sure you do it at the right points in the song. In Co-op, it is almost always best to do it when you get it as otherwise you will run over the top of other phrases but do make sure you do it on sections with lots of notes as these will get you more points. Another tip is to only do it when you both have 4x as you will maximise your points that way.
Hope this helps and feel free to send me a message if you want any other advice.
