Playing through The Taken King story missions as a Titan will eventually unlock this trophy. When you get to "The Burning Shrine" mission on Mercury, you complete "challenges" using the Sunbreaker class (it's really just destroying a ton of Vex with the Hammer of Sol super). It's a pretty straightforward mission that takes about five minutes to complete. Just follow the red blips on the mini-map to find all the Vex and finish them off, and you'll be good to go! The trophy pops after the mission is over and you go back to talk to Zavala at the Tower and complete the mission (accepting a Titan mark, make sure you have space in your inventory).