In The Devastation, you can take advantage of the "war" that goes on constantly in the zone as some mobs are ready-to-attack other types of mobs...like Spiders don't like Undead, just like Mantisknight said.
As far as damaging these mobs and then trying to get the last hit in so that the corpse is still there as one of these zone shifts occurs, tried that...didn't work. They caught on especially in this zone.
The easiest way to figure out what mobs will drop the skins are the mobs that are red, as stated above, and who list as 73 or 74 on your HUD if you have one that displays that. The typical named generally do not drop the skins. Most of these mobs summon, however some were easily solo'd on my Necro.
There is an instance that does drop the skins, however its much easier to get them from the mobs in The Devastation. Sometimes, I even found it better to have someone there to tag the mob out of the train as they can get very close together, but they don't tether. The Bolvargs near the rim of the zone do tether though and its a good idea to stay away from them.
-TurkReno