On maps I've found, DW is shown to spawn along the west wall, just a bit south of Dracolich, but I'm not seeing any Decrepit Toads that pop anywhere near there. Is that it's confirmed PH? Does it pop somewhere else?
I believe the PH's for this mob vary a bit, but the map location is correct. I found a couple of decrepit toads wandering in this area, killed them and waited for respawn. After 30 minutes or so I realized no toads were appearing, but there were 7-8 "a putrid fiend" which I wouldn't expect that many just as PH's for Irak Altil. I killed the wandering putrid friends, concentrating on the ones farther away from the western wall. Sometimes the mobs would respawn as decrepit toads, sometimes as putrid fiends, and sometimes as "a delirious samhain." So my advice would be to kill any mobs in the general area that have been established as named PH's in other parts of the zone, and eventually you will get the Decrepit Warder.
Can't see where he casts anything. Maybe a stun at one point..not sure. He hit me once for 11k, most were 5k and above. Mean warrior thing I'm assuming.