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