Practice it a lot. It's just 4 note runs over and over again, which basically becomes a smash of those four keys. If you practice enough it becomes second nature. Now, I haven't practiced it enough, it's not yet second nature to me, so I can't do it every time. I've only 3-starred Raining Blood, because the time I got through the Thrash part fine, I wasn't too set on the repeated greens for some reason, which are now easy to me.
Apparently I had also not yet five-starred Knights of Cydonia. That is no longer the case!
For those who are having trouble getting streaks on the septets (the quick notes), well, they're septets, which means they break the natural rhythm. What I ended up doing to hit them was holding the strummer deeper than usual - instead of with the fingertips, with more of the lower part of the fingers. With a better grip on it, just basically shake your whole body. It worked for me!