Broken Cerberus

I have come across some ridiculous bugs in my time, but this one takes the cake.

Platform: Xbox 360
ISP: Comcast
Type of Internet Connection: Cable
Internet Connection Speed: Not sure.
Date & Time: Friday, August 5th 6:00AM~8:00AM
Frequency: Always
Character Name: Zatias
Race: Elvaan
World: Asura
Class/Skill Rank: 90RDM/WHM, 90BST/NIN
Area and Coordinates: Mount Zhayolm past the gate where it always spawns
Party or Solo: Party and Solo
Monster Name: Cerberus
In-Game Time: Ranged from Iceday all the way to Lightsday.

Steps:
1. I was going to do my earth weather trial in Mount Zhayolm so I went there as BST.
2. Since I had widescan, I checked to see if Cerberus was up so I could play with him. Turns out, he was up.
3. Went past the gate and engaged him. Pet attacks were doing ridiculously low damage (10-30 max) so I went and hit it, my melee did around the same, slightly lower.
4. I warped and my friend and I went back out as RDM and NIN/WAR.
5. Katana melee doing 10-15~ with Blade:Hi doing 80-250 max. After 20 mins of melee like that we warped as cerberus was still standing strong around 85%.
6. I remembered that Atonement has enhanced damage against this type of enemy, so we came back RDM and PLD/NIN and defeated it after 70 minutes of fighting. Atonement was doing 750-1050 damage a pop.

I thought we were just weak as hell, but I looked around and saw threads talking about how its damage resistance has skyrocketed since a certain update (around the release of Heroes of Abyssea), yet GMs say he is "working as intended".

Is he really working as intended? Because this damage against him is quite ridiculous, and so is the 1k stoneskin you have to break everytime he uses Magma Hoplon. (He used it over 20 times by the way)