Story related and cannot be missed.
Once you defeat the boss of level 7 the achievement unlocks during the cut scene that follows. Use Swampfire for the whole fight. First he flies around shooting at you while you get attacked by regular guys. For this phase you need to use and when he stops in the center. It will cause an explosion and damage him. This should only take three times. After his ship crashes, jump across the debris and take out his 3 guards as quickly as possible. Now unleash with some more fireballs to blow up his shield. When the shield is down run in and punch him. Eventually his shield will come back and so will his guards. Just keep repeating the steps until he is gone. Don’t forget to play smart and save your pots for when you need them. They all contain health power-ups so try not to destroy them until you need to.