Blackrock Elementals - do they still spawn, or are you just playing with the remnants of the event spawn in Ilshenar?
Blackrock eles are essentially peerless valorite elemental paragons (only without any decent loot), and as the valorite eles share the abilities of all the other ore elementals, it means they have the Shadow Iron elemental's immunity to magic and pet damage.
Shadow Irons are easy enough to drop with a weapon (such as a mage weapon that's an elemental slayer).
Valorites will kill you with reflected damage in melee, but can be killed by earthquakes and arrows from a bow (such as a mage weapon, if you're a mage)
Blackrocks are so powerful you just run away.... Other than getting guards to kill one, the only other means one has ever been reportedly killed, was using a legendary bard, who had TWO present, discorded 1 with an elemental slayer instrument, then had the stronger one kill the larger one with provo.
Do note that UNTAMED creatures can hurt it - if you can get it to follow you into an area where creatures will attack it, they will damage it (imagine what would happen if you got one to chase you to Oaks -it might work the spawn FOR you)