GoS The Oldest Trick In The Book.

Raulins, one of Ingo's two lead instructors, has shown you how to throw a handful of dirt/sand etc. in to your enemies face as a Bonus Action.

The recipient is blinded for the rest of the round. 

This technique will only work once per combat, as the enemy, and any who observe such a dirty trick, will be weary of it for any subsequent use.