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.
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The Weapon Can Be Evolved With An "Empty Tome" At Level 5.
- Five Empty Tomes Are Present On Space 54.
Also, Get The Weapon To Level 8.
Achievement Unlocks At The End Of The Run.
Good Luck!