Nothing special needed. Just keep distance from him after he breaks his chains and use long range weapons like the stakegun.
Shoot the bubbles under him until you get permanently burning bubbles (burst them all). Shoot bubbles that float near the boss. Shoot at the boss directly until he has extinguished all. Repeat.
Destroy his warhammer, then Guardian himself.
No special tricks needed. At the end you need to destroy the statues when he's drawing energy from them.
"Push" (shoot, really) the meteors he summons back at him.