In Mission 4: Chapter 1 - Into The Abyss, after you encounter the first Titans in this mission, a gate will open up and three Titans will spawn (approximately 2-3 minutes into the mission). Damage them until they have very little health remaining and will die with 1-2 hits. Damaging them automatically powers up your core abilities. Use the Burst Core (Expedition loadout) to kill them quickly and get the "Hat Trick" achievement. If you fail, restart the checkpoint and try again.