I've been going at this achievement for a couple months now, and I seem to have the worst luck ever. The achievement describes that you have to kill at least 4 out of the 7 rare spawns in Silverwood, and I've killed 3/4: Brod, Mok, and Marlrog.
The last 4 that I have the option to get for the achievement are Pandora, Pentheus, Werewolf, and Vengeful Spirit, which I looked the waypoints up for to speed up my achievement progress.
The problem I've had with this achievement is two things: one, the little island Pandora is supposed to spawn on (7113, 3467) always has a Defiant foothold on it, so if Pandora were to ever spawn, he/she would immediately get killed my the foothold's members, and second, the spot Vengeful Spirit is supposed to spawn on (6353, 2641) almost always has a rift on it, therefore limiting the chances of someone ever going to kill that rare spawn because the rift monsters will kill it.
This leaves only 2 spawns left, and ones I have yet to find.
I understand that this is a dumb achievement to be arguing about, but the spawn rates on the foothold and the rift in those specific areas should be fixed. Thank you for any help in advance!