Easiest way for me was;
Play on Act 1, Chapter 2 ~ Abandon Ship.
When you get to the Leviathon fight - you are ready to unlock. You can restart the checkpoint as well, so it's no fail!
You will have the option of using the Silverback - do this. It is NOT a loader but it DOES count.
On Casual, once you pop the Leviathon's eye, loads of polyps spawn - back-up a bit and hold B. You will SMASH the ground and either kill via contact or shockwave - stay alive for the fight and you will earn in 3 mins. Easy!