Per comments below - you will get this as you play through the game as there are plenty of undead; however, you can grind this before the tutorial is complete! Took me a little under an hour.
After you have collected the arrows for the archers and right before you head to the Sleeping Dragon Bridge; there's a truckload of Undead Soldiers that constantly respawn, just move in a circle around the battlefield and kill 'em all.
You can also go back down past where you picked up the arrows and there's a ton of skeletons there - one whole 'loop' like this will net you 50kills.