You'll most likely get this over the course of the game, but there is a great spot aobut a third of the way through that you can abuse to get this trophy quickly. After beating the Precursor Robot and arriving in Haven City the first area you get into will have an infinitely respawning supply of the small scorpion-like Metal Heads you've been seeing that die in one shot/hit, and about half of them in each wave will have Skull Gems for you to collect. Just keep killing them until you get the trophy.