You can get this by playing on offline local mode with CPU since all the characters are unlocked there, select infinite lives and the max amount of time in the stage, select Marvin the Martian as your player character and Iron Giant as your opponent due to his big hitbox, if you want a lighter character to make it quicker, i'd say Jason is the second best choice, i recommend a stage that doesn't have a whole lot of platforms like the Sky Arena from Steven Universe, use Marvin's aimable projectile ability right next to Iron Giant or Jason and keep shooting upwards until they're dead, keep doing this until you get the achievement.
This also serves as a way to get the achievements for Projectile Kills and Upward Kills