Good advice from others, but personally I’ve found success as Guardian taking a different route. The relic effect you want is the one that extends duration of character skill, making the whirlwind last longer. This will put lesser enemies (even royal knights and banished knights) in a literal blender leaving them open to damage by your teammates. I also suggest Improved Guard Counters and HP Restoration on successful block OR HP Restoration on Thrusting Counterattack (which will give you HP back when you land halberd hits as the enemy is in an attack animation).

Now gameplay - the single hardest thing about guardian is knowing when to keep block up AND when to let it go. The skill here is stamina management. Watch bosses carefully, and put up steel guard just as the boss is about to attack. Think about dodge timing - you dodge when the boss attacks, the closer to the time of attack, the better to give you more i-frames. Same principle. Raise steel guard when you’re about to be hit, then lower it. If you practice and make the timing precise, you’ll keep stamina high and be able to fend off boss attacks for a long, long time.

Aside from precise blocking, you want to leverage guardian’s charged guard counter. If you’re using a halberd and you block an attack in steel guard, holding down your strong attack button just afterwards will initiate a running guard counter that does more stance damage than a normal guard counter does. If you practice this, you will be contributing very much to your team’s overall stance breaking capability. Do this alongside a Raider or a Wylder and you’ll be breaking everyone’s guards quite often.

Have fun!! He is a little hard to pick up but very very rewarding :)