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