Available during "The Battle of Hulaoguan Pass" which happens during Part 4, at a recommended level of 37.
There will be many different ballistas available to you throughout the mission, but the easiest and fastest is to get on the one nearest the initial battle flag. Run to the ballista, kill an enemy, and then either visit the battle flag to reset the enemies, and die on purpose to respawn, and repeat this process a total of 5 times.