The guardian is one of the 3 bosses that you can encounter at the end of act 1. You can actually see which boss you are going to face by looking at your map, as the big top icon represents it.
It has 240 HP, and it will start in offensive mode. If you deal 30 or more damage to it (actual damage, such as physical or poison, but not blocked damage), it will switch to defensive mode. Your main goal will be to switch it as soon as possible because it hits like a truck in offensive mode.
Inflicts 2 vulnerable + 2 weak
When switching to defensive mode:
Immediately gain 20 block for the turn
Sharp hide: this ability deals 3 damage back to your character every time you play an attack card, regardless of the damage inflicted or blocked, or the number of hits provided by the attack.
8*2 damage, removes Sharp hide and returns to offensive mode
*have enough HP before the battle so feel free to rest at the previous campfire
*remember that its first move will be to block so you have 2 turns to set up your powers / poison
*if you deal 40 damage before he charges for its 32 damage attack, you won't have to suffer much yourself
*All status alterations works great against him as he has no artefact to start with.
*Its sharp hide will deal significant damage if you only rely on shivs (or multiple low cost attacks), remember to block before attacking when it is defending
*feel free to use potions for boss fights if you have leftovers, they can help quite a lot
*I'm not great with the Defect so I can't really give advice for this character specifically. Poison Silent and Strength Ironclad archetypes were murdering him reliably though. I suppose defensive archetypes would work as well, because if you can tank the 32 damage (with Ice Defect for instance) you can take your time dismantling it piece by piece.