After defeating a Guardian for the first time, you can often re-challenge them at their original spawn location. While they don't offer story progression rewards, re-challenging Guardians is an excellent way to farm Skill Pieces, which are essential for unlocking and upgrading Sonic's abilities. It also provides an opportunity to practice your combat strategies and improve your parry timing against specific enemy types. Look for the glowing markers on your map indicating a re-challengeable Guardian.
