How in the did you do that?
Here's my build:
7/13 arm
Grave Clod
Ice Shield
Sanguine Cannon
Any lightning attack
Tri-ice Roots *
Poison Regular Golem *
All offensive sigils (+50% atk -25% def is great) including freeze amp.
I only marked stars on the offerings that are mandatory to boost.
Run past the first cats; kill the two rats with counter hit shield and sac/save them so the third cat doesn't get to eat them.
Skip boss cutscene and summon the golem so that it does not knock him down on summon, but it can hit the boss while it's roaring.
Block the first dash attack so that its back lands in the golem's range, but not its heads.
Spam Grave Clod on his head until he gets up. He will roar after getting up, so continue to spam 2 more. If you run out of Grave Clods, use blood instead.
When he charges again, block again to knock him down.
Spam tri roots in his face to freeze him. You need the * so you have just enough casts to do it. May need ** if you don't have the chaos arm upgrade on freeze amp.
Summon the next golem while he's frozen, then hit him with the lightning attack to knock down.
Bring him to red HP with your blood, then use your black rite.
Easy, right? =)
Edit: Some general non-build specific tips:
The stun time when you block his charge gets lower and lower each time. Pull out your biggest guns first!
When he's charging his lightning, roar, or homing lightning of doom attacks, you can interrupt him with a golem summon if you're quick.
You can hit him right before each projectile of his double projectile attack with quick spells. You need to roll immediately after to dodge the actual attacks.
If you have no choice but to fight him legitimately, the stone armor near the rat spawn helps a lot. Seems to halve damage and increase your stagger threshold (the latter may not be true).