The Phas3r Can Be Found On The Stage "Space 54".
I Used Queen Sigma, And Selected The "Mad Groove" Card.
- This Brought The Weapon Right To Me.
The Weapon Can Be Evolved With An "Empty Tome" At Level 5.
Also, The Weapon At Level 8.
Achievement Unlocks At The End Of The Run, Once You've Successfully Gotten The Weapon To At Least Level 7.
Good Luck!