i just did this ..got all 20 in about 6 hours over 2 day period.... -500, -2300.....they are on a 16 minute respawn
how to get there ... when u enter the zone run (invis... nothing argo the whole run) to the north side of the center building enter the building and run up the ramp to the east sky bridge ... once u pass the doors and enter the sky bridge ... the first intersection has 6 mobs ... there are 3 mobs on each side 2 sentinels and 1 drake... and there is only 1 wandering drake ...
breaking camp ... what i did was kill the wanderer first ... if u stand die center they others dont argo ...standing close the wall enfront of any sentinel using arrow i was able to pull just 2 a drake and a sentinel .. then killed the last sentinel ..same on the other side ....the 6 mobs are static ...
Warning ... they run at 20% and will argo anything along the way ... root or snare works well..
the strong spawned about avg 1 every round ... if i got none one round 2 would spawn the next or the one after that ... camp was pretty quick considering.... good luck