Kill the Guardian and collect the Firewalker Shortsword for your troubles.
Don’t knock over any of the fire goblets if you want to fulfil the bonus requirement. The theme of fire-based attacks in this portion of the game carry over to the boss. Almost all of his attacks are fire-based, so you might want to consider whipping out the Harmony Buckler Shield with a couple of Lesser Fire Runes to grant you 100% defence against fire and physical attacks. In terms of battle range, once again ranged is advised.
When the Guardian kneels and the fire goblets start to glow, get to the outer edge of the arena to avoid the balls of fire that have tracking capabilities. If you’ve opted for heavier armour, you’ll need to utilise your dodge instead, as you won’t be fast enough to just run away.
If you aren’t particularly fussed about the bonus requirement, you can charge the goblets, knocking them over and causing the Guardian to run over and light it again and giving you a brief opening for an attack. When it reignites, it’ll explode, so you don’t want to be standing too close.
Use your Gauntlet at range, avoiding his 2-hit combo and rush attack that you should be able to dodge fairly easily. He’ll also rush you immediately after the fireballs come for you, so be careful. Attack after his combo and dashes as this gives you enough time to ensure your attack makes contact.
If you’ve chosen to equip the Harmony Buckler Shield, you should be just dandy for close quarters combat. The Guardian will use the old 2-hit horizontal combo that you definitely want to avoid, as it’s near impossible to recover when knocked down and you’ll most likely die under a barrage of follow-up attacks.
He also has a sexy little routine involving wedging his sword into the ground and swinging around it like his tips depend on it. This is preceded by a powerful kick, so just be ready to back up. And maybe find a bundle of ones.
Once you’ve defeated him, collect your reward in the form of a Big Sealed Rune and Firewalker Shortsword.