The Vanguard is a melee enemy type encountered in Chaos 1 difficulty and above (Except Chaos 2). It has relatively low health and deals a small amount of damage. They carry a shield which blocks projectiles rendering them useless.
To deal with Vanguards you will want to place Auras, Traps or Nodes but you can use some Towers that are not projectile based like Ramster or Flamethrower. It is also possible to place Projectile Towers on the sides of the lane aimed to hit them in the back but it is generally harder to do so. A Piercer mod can be used to bypass the projectile block at the cost of reduced damage.