Fought one in a group, and these buggers charmed our Monk and mezzed me (20 warr) and our Cleric! I think they must be Enchanter/Cleric hybrids. Be warned!
"Nisch" gnolls are always clerics. You should almost always attack them first, and try to keep them stunned so they do not heal. An enchanter is very useful because they can mez the other mobs while you kill the nisch.