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