ATTENTION : You need to Hail him (default Key is "H" in order to engage him!
During the fight you ll be sent in a barred cage, fight the flames spirit that are burning you and then send your merc (or another player if you're in a group) to kill Flame Lord Thyr (You shoot at distances too. This worked for my ranger. --Eddardson).
Once defeated, you ll be freed and can enter Solusek's Eye