That one in mire is higher level than the two in reef. The two in reef are on the NE section of the map you enter from Azouph Staging point (the common imp/fly exp one) You'll need the once a week key to get to them if you take that route, and it's a long run from Dvucca out to them.
The first one you can easily pull south to a section where there's no other mobs, and no worry of aggro. You just have to wait for it to wander away from the sight/sound/low HP aggro VT-IT Lamia Idolators.
The second one is through the tunnel to the east, in a room by itself.
I've killed the one you found, and killed the two in reef, and the ones in reef resist a whole lot less. I don't think I ever had a sleep II resisted by them with 267 enfeebling, although they resist sleep I a decent amount. The one you found resisted a chain of about 5 in a row sleep(ga) I-II spells from me.