The fight takes place on Wenshaganaw
It's by the Question Mark (Camp near the water) at the northern part of the map, just below the northern most exit.
Hopefully by now, you've explored alot and found quite a few skill point shrines. I was also Level 16 when I killed this beast.
The trick is to stay out of reach of the guardians pounce attack, this is much harder to avoid than the guardians magic and can result in instant death.
Hopefully by this point you've also Unlocked the "Bomb" skill.
Use your rifle at distance to destroy it's armour. Unleashing bomb when you have full rage (L1 and find it under 'spells') DO NOT USE L2, he'll just throw you in to the air.
Use a few poison bombs from the techniques section of the tactical menu too, this seems to make him less of a threat.
I took Kurt with a ridiculously OP hammer that I crafted to have huge shield damage and Petrus as the guardian seems to completely ignore him when all the hunters are dead. If Petrus dies before the last hunter falls, use a resusitation powder.