Here is the full list of all 12 Mighty Gunvolt achievements.
Clear all 9 stages with any character
Obtain 9 lives
Defeat a boss without taking damage
Reach a boss without defeating any enemies
View Beck's ending
View Ekoro's ending
Befriend 10 enemies in one stage with Ekoro
View Gunvolt's ending
Discover the hidden butterfly
Obtain more than 30,000 points in a single stage
Get a x1.50 combo
Defeat a boss