[Hadranox Denied] Help!

Hello, I've been trying to do this achievement, but I fail everytime. I read several tutorials like paladins with bubbles, rogue vanishing, hunter feign deeath, we tried everything but even if we manage to reach the tunnel without entering the combat, Hadranox is still below, since the first moment, eating creatures.

So no matter the burst, the creatures will keep spawning and healing him.

I thought the creatures started spawning and go to him only when engaging the combat with the big guys below, but it turns out as soon as we enter the istance, Hadranox is already down there "in combat" with bugs coming from the doors, no way to reset it. Even if we die and re-enter, everytiem we reach the brdige we can see Hadranox in combat with the mobs.

Any tip? Is this bugged?

Thanks

Did it on random rhc as tank. What I did was to tank adds on that narrow corridor, away from the boss, while the rest killed the boss.

Another thing to note that you may or may not already know;
Hadranox heals from damage it does to enemies from the DoT and when they die while they have the DoT debuff on them.

Everyone in the group should be using as little AoE as possible and focusing on using single target DPS spells and abilities so the adds die as little as possible so they dont heal the boss. It shouldn't be to difficult to take down the boss and get the achievement if single target is the priority, unless the dps is severely lacking.

Did it yesterday.. If you have a priest / mage in your group.

Don't bother running down the bridge ( which you see as you enter after clearing the gatekeeper. ) .
Just jump down to left with levitate. so you are as close as possible to the boss.. then SINGLE TARGET the boss.. AOE damage kills the small mobs around it and might heal the boss.. its easy.