Guitar Hero III
59 Achievements
1,000
Xbox 360
Half Mill' Club
Score 500,000 on any song in a Co-op match
10
How to unlock the Half Mill' Club achievement in Guitar Hero III - Definitive Guide
I recommend playing "One" by Metallica on Medium for this achievement. Again, it's all about the practice. You should be able to get through at least the first half of the song without missing a note, as the beginning is relatively basic if you've seen it before. You and your friend should be well on your way to half a million then.