Anguis epic dragon in antonicaFollow

Ok so last night there was a raid forming to kill the epic dragon anguid in antonica. after finally getting organized we have a fairly balanced raid with 12 people level ranges 23-30 (about 6 people were level 30 or higher mentored down to 30).
So we thought.. Hey we will own this guy pretty bad with our group.. as i have fought him before with my healer and we had lower level people.

Anyways the fight starts.. And "Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune Immune " .... Basically only the mages were doing dmg.. Anyone that was melee wasnt doing squat.

needless to say... without us melee doing dmg.. The healers and mages ran out of power.. and we had a full raid wipe.

I was trying to look around at different websites.. BUt i cant find anything that says he is immune to stuff..
Why was he immuning everyones melee attacks? (BTW I had slashing weapons, and the tank had crushing... Both of us handcrafted quality) he was immuning both of us.
Any idea why?

I have run into this problem with some mobs before mainly wisps and the such. It wasnt that they were immune to physical damage it was that you needed to have a magical item (one with any stats what so ever) in order to damage it with physical.