my gues is the Quest item does not tag the eye properly for it to register as your kill.
so now the trick is to stand on one of the pillars as shows in the screenshot . http://imgur.com/a/9dAnp
make sure you LAND on the pillar as show here before you dismount, or else you will fall down and DIE! http://imgur.com/a/GtxGF
hit the Eye once or twice first with a spell or autoattack (so you tag the Eye as your target)
after that you use the quest item to bring it down as low as you can .
now the last bit of hp the Eye has you get down by just using attacks and spells.
i dont kno if you HAVE to kill it manually , but thats how i got it to complete.
this might be a litle bit hard to do at points becouse allot of other people around you will use thier item and kill it .
make sure the Name of the Eye is not grayed out when you are killing it ! if so it will not count as your kill .
this method worked for me 09-08-2016 12:33 (realm time)
i hope it does for you . have Fun.