This takes 750 points to unlock. You can earn points grinding away on the lowest difficulty, but Higher difficulties are faster. However, if you don't get at least 80% of the Greys, you may lose points.
I recommend practicing on the untimed mode until the levels are familiar, then moving on to the timed mode. If it looks like you won't get enough Greys, restart to avoid losing points. The levels are partly random, so I can't give you much more help.