Are you running out of his AoE? That may be the trick. Stay in and have your healer heal through it. The tank doesn't take much damage, and the group takes 0 aside from the big AoE. Your healer should be able to throw a little dps in between preparing and dealing AoE hits.

I've completed it with all VR2s.

Yea it's quite hard, but definetely doable in that range.

Key is correct skill use, not gear, nor stats. Most of us had greens/blues, not dps focused stuff either.

KEY SKILLS:
All classes:
MANA POTIONS ALL CAPS. You must drink them during the fight for more dps.
+Hp/magick Food - more initial burst dps and easier on the healer. (I maxxed our provisioning for this reason alone, it's so helpful - i hand out food to everyone in my groups free).
Mage Light - Preferably Inner light: +22% spell crit? YES PLEASE.
Templar:
Rune Focus - this regenerates everyones mana allowing for much better dps.
Biting Jabs - best single traget dps ability.
Tank:
Pierce Armor - Anyone with a sword and shield setup should use this.

Can't give a lot of insight on the other classes best single target dps as I don't recall atm (asked friends, but yea meh forgets)

But pretty much know they are all 100% certainly CLASS SKILLS. Never use weapon skills - they are garbage.

Strategy:
Everyone stand DIRECTLY INFRONT of the gargoyle. Sound counter-intuive ? Sure, but it works the best by far. As it allows everyone to be inside the rune focus (and/or fighter guilds circle). And the boss does not cleave. Tank needs only to taunt appropriately, and block power attacks. Rest is 360 aoe anyways.

This game is all about magicka, spell damage and magicka regen. (And not dying). Nothing else matters.

I've also a video of VR2-4s doing it, in about 80 seconds (much faster then neccesary). Though honestly my interest youtube these days is mostly lost, so probably not editing it/uploading it. But yea looking at it, seems the time limit is about 120 seconds, so we did it vastly faster the neccesary.

Most runs with weaker dps players, were quite close, often in the 110 second range.
Even manged some with deaths in the 120 secon range - with rocks RIGHT on us.

So i'd say it's tuned correctly. Just tuned for players who REALLY put the effort in to use the best skills. Great gear not required.

ya like someone else said, stand in the aoe, make sure your ult is charged (DK banner is godly for this), spam your highest single target skill as much as you can. For sorcs (which I main), you want to blow your magika bar during the non-aoe phase, then channel dark exchange during the aoe - especially if you have the armor/resist morph (which any intelligent sorc should pick), gives you resource and helps the healer out a lot.
Use magika+spellpower (or spellcrit) potions if you have them (or find an alchy that can make them... hint hint pm me in game lol).

Killed him with a team of <VR3 (I am VR2, was VR1 at the time) once people figured out the strat.

Animation canceling is an amazing thing. I personally noticed my dps nearly double at vr2 by learning this. Basically you can cancel a light attack with a skill and cancel a skill with a block. If timed right you will get off a light attack and skill faster than a normal light attack. It really comes down to everyone knowing their class's and requires some prior testing to pump out the most dps.

My regular guild group kills him pretty regularly now. VR 2-5 group. I'm the tank, and for this guy I taunt, then immediately swap to 2-Hander and lay out dps. We stay in his circle at all times. Healer does 50/50 dps/heals. I keep up molten weapon buff for group. At 25% I forget taunt and just spam Execute. Chug those stamina pots! We kill him with time spare now that we've had some practice.