Blackrock elementals have about 3000 hps (nowhere near what peerless have) and moderate resistances (40-60 in each).
They also have a few special abilities:
They drain life from every living body within a radius of them (I think it's 10 tiles).
They are completely immune to magic.
They are completely immune to pets.
The only way to kill them (AND I MEAN ONLY WAY TO KILL THEM) is to have the fewest amount of bodies near them when they drain life as possible.
NO ENERGY VORTICES! They do no damage to blackrock eles, and provide another body to drain life from, making it exponentially harder to kill him.
The best way to kill him in my opinion is to have one well-outfitted dexxer stand and tank him, and have 3-5 mages EXACTLY 10 tiles away from the dexxer (and 11 tiles away from the blackrock ele, so they don't get drained as well, but can still cast on the dexxer) casting the occasional bless and heals on the dexxer.
I'd suggest no archers, because they would have to be within drain radius to hit him anyway, which would be counterproductive. The ele would just run down the archer, and in turn drain all the archers and all the mages, filling it's life back up and causing pandemonium.
If you have enough mages to keep two dexxers alive, put two dexxers up there. Double the drain, but double the damage, making the entire process faster.
This is kinda hard to do with various other guilds there casting ev's left and right...running up and aggroing the ele and running off (making it hard to control where he's going).
So that's what it takes to kill him, but we (Sonoma as a shard) lack coordination, so I won't be there when they finally fumble around and kill the first one.