Story related and cannot be missed.
Once you defeat the boss of level 4 the achievement unlocks during the cut scene that follows. To defeat the boss, start by blocking its attack. Then use Ampfibian to charge the middle platform. Quickly block and then run forward and switch to Big Chill. Freeze the dragon and then switch to Swampfire and shoot the head with fireballs. Repeat 2 more times. Now the dragon will start flying overhead and you will notice signs on the 4 corners of the ground. Switch to Ampfibian and cross the lava rocks to the sign that lights up and shoot it with electricity to fry the dragon. Repeat until it dies.