Playing on normal while keeping a high multiplier should give you a good amount of points. Scoring 750k is a bit more difficult than 500k, but it's feasible. I was scoring 1 million several times on normal and I missed notes at times. Try to pick slower songs to correct your pitch if your off.
Some Suggested songs:
Start of Something New
When there was me and You
You are the Music in Me
Can I have this dance?
Other suggested songs are welcomed, just write them in the comments.
Just a reminder: You only need to do this 10 times. Since there are 25 you have a wide variety to choose from.