Once he's damaged to about 10% of life left, he respawns as non-KoS version, to which a ranger at the right point in the epic 1.5 can hand in the Red Dogwood Heartwood and receive the Wind-blessed Heartwood, at which point the non-KoS version despawns and a chest can spawn. If he is attacking a merc at 10%, just let it kill the merc and then he'll respawn as the non kos version.