a stone ward  

these 2 guarding entrance to Guardian of the Seal and Echo of Nortlav are not lvl 55. they were consistantly red to my 55 chanter after hours of killing them. they were blue to a 61, so must be 58-60 range. Yes they are very magic resistant even with tash, poison based dot from my beastlord seemed to land fairly well, and i could land my clicky burst of fire most all the time. Very short respawn timer. gotta be 6 min or less, i wouldnt say 3 min though. i could usually kill the 2 Stone Wards one at a time, then pull a broken mob from up the tunnel and then hit them respawning again. If doing the Guardian, you pretty much will have these guys up while trying to finish him off and they will agro when he knocks you back to the area near them.

Two guard the doorway into the Guardians' and Echoes area. They have a 3 minute spawn timer (yes, three), and fairly large aggro range. If fighting the Guardian and his AE pushes you anywhere near the door, expect two adds, likely a golem or two also.