This Spirit set me back a few days and caused some frustration.
The first time I went after it, I turned in my seal to Janomin and ran my 52 beastlord(with dps merc) and 75 ranger(with tank merc) to the bottom of the king's room, and the Spirit came running through the giants' door on the left. He wasnn't any harder to dps down than the others, but when he started running after 20% he managed to make it all the way to Lady Vox before I noticed he left and went to find him. My dps merc was killed by giants, my beastlord was killed by Vox, and the tank merc killed the Icebound Spirit but I didn't know where his corpse was. My ranger was banished once she was in the vicinity of Vox. After running both characters back to Permafrost to search for the corpse, I wasn't able to find it before it rotted, and the scout had already despawned.
About 3 days later, I went back knowing better than to fight him on the lower section of the king room. I took the same characters and mercs, and left them at the top, just after the 2 grates. I marked the ranger as Puller, went down to just before the giants' door, and the Icebound Spirit came running through. Keep in mind that he will not summon until he's damaged. I ran him back up to the top of the room where the other characters were, and took him out pretty easily. Note that he did not run down to the giants/Vox when he was on the upper level of the king room, he instead went back toward the entrance.
Unless you can thoroughly dps that sucker from 20% to dead in no time flat, I recommend using this method and just pulling him to save some tears.