The Geode is a support enemy type encountered in Chaos 3 difficulty and above. It has moderate health and deals no damage. They have a large pink shield around them the will reflect any projectile in a random direction and can cover many of its allies. Towers inside this shield are debuffed making them do slightly less damage.
To deal with Geodes 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 near the spawn to get them inside the shield but it is generally harder on yourself to do so. A Piercer mod can be used to bypass the shield at the cost of reduced damage.