Story related and cannot be missed.
Once you defeat the boss of level 1 (Enoch) the achievement unlocks during the cut scene that follows. To defeat the boss, kill all the enemies that are around you and be sure to block the red bolts that the boss shoots at you. Once the enemies are dead, switch to Humungosaur and push the block into the base of the platform the boss is on. A platform will appear so use it to get across to the next section. Kill the enemies, block when you need to and push the next block into place. You will have to destroy the debris that is in the way first. Once the 2nd block is in place, another platform appears so hop over it to the third section. Destroy the enemy, block what you need to and use Echo Echo on the green switches to open the gate. Then switch back to Humungosaur and push the final block into place. Enoch will jump down to your level, so just pound on him with your regular and special attacks until he falls. If you have trouble, then don’t forget to upgrade your alien. You should be able to max out the current level of stats for Humungosaur at this point and he will fall in about 30 seconds.