The Javelin Thrower is a ranged enemy type that throws piercing spears, these are typically thrown at the bottom of blockades causing it to hit the ground afterwards. It has decent health and deals a moderate amount of damage.
Like other ranged enemies placing defenses that have long range or placing traps and auras in the lane will kill them before they get into range to attack. If you have Series Ev2 you can use her Reflect Beam defense in front of a blockade to bounce the projectiles away. They die relatively easy to any damage done to them.