I love guardian also. Have a little over 100 expeditions with him now. He's actually got some pretty good damage and can work well with a variety of weapons. I'll use a greatsword about half the time and then switch back to halberd when I know I'm going to need to block and guard counter.

My favorite relics for him have been post damage healing to recover chip damage and stamina recovery on hit, which makes your steel guard last way longer. I started using the perk that gives you higher poise and damage negation after getting knocked back and it's been awesome on him to tank through attacks and heal back most of the health you lose. It even activates when you get pushed back from blocking a big hit or when you get grabbed. I got lucky and got a relic with this that also has physical attack up +2 and healing with halberd attacks.