Here is the full list of all 31 Magnetta achievements.
Beat the Chemical Plant stage
Beat the Magnetic Lab stage
Beat the Swamp Station stage
Beat the Robot Factory stage
Beat the Metal Mines stage
Beat the Launch Complex stage
Beat the Holographic Facility stage
Beat the Teleportal Testing stage
Defeat Jetso
Beat the game
Beat the game in Speed run mode
Beat the game without getting a Game Over
Collect a circuit
Collect all circuits
Beat a stage without taking damage
Change your hair color
Destroy every enemy type
Defeat 400 Enemies with the Arm Cannon
Defeat 25 Enemies with the Power Drill
Defeat 25 Enemies with the Magnetic Pulse
Defeat 25 Enemies with the Robo Grapple
Defeat 25 Enemies with the Seekor Shot
Slime 25 Enemies with Swamp Slime
Kill 5 enemies while standing on a chemical platform
Access the Soundtest
Beat a Glitch stage
Speed run the game in under 1:15:00
Blast an enemy through a Teleportal
Beat all Glitch stages and rescue RENOVA
Beat a Challenge in Challenge Mode
Beat the game in easy mode