As you progress through the main quest line, you'll be tasked with a mission to locate a boat fairly early on in the game. Once you acquire the boat, you need to hold down the X button to use the speed boost and run over any undead you come across. They can be in the water or on the shores of land, as long as you run them down with the boat while holding the X button, you'll make progress toward this trophy. If your doing this correctly, the game will periodically pop up an in-game progress bar to show your progress toward unlocking the trophy.
The following video made by RetroMarceline will give you a visual idea of what needs to be done. All credit for video goes to RetroMarceline