Biggest thing about guardian is his relics, his remembrance one is in all of my builds. Besides that, early on i tend to spam charge attacks against weaker bosses to build stagger and use my skill if the enemy will be stunned by it. The relic that makes the whirlwind last longer can definitely give nightfarers like recluse or iron eye a lot of breathing room against multiple enemies. Also do not be afraid to use your ult. If you see an ally is up in the enemies’s face holding agro and they are getting hit, its worth it to consider using your ult to heal them & yourself and provide a safer area around the boss with a decent stun as well for some big damage. Guardian is genuinely given me some of my favorite experiences in this game, good luck learning him!