How much would Cotton Guard raise the user's defense if its ability is Simple?
It would raise it by 6 Stages Which would immediatly Max their defense.
But There are currently No pokemon who have the ability simple and can learn Cotton Guard, the only way to pull this off would be to have a Team member in a double or Triple battle use Simple Beam on you.
Instead of raising its Defense by 3 stages, it gets raised by 6 stages. You would need Simple Beam and Altaria or Cottonee/Whimsicott to actually achieve this.