Here is the full list of all 8 Synth Riders achievements.
Complete 5 songs
Complete 3 songs
Reach 1,500,000 points on Supress It song
Reach 1,000,000 points on Waveshaper song
Reach 700,000 points on Stilz song
Reach 600,000 points on Overvad song
Reach 500,000 points on Sunset Hologram song
Reach 300,000 points on The Curse Song