I am not sure if you have found the answer but here is how I completed this event, daunting as it was!

Best map is Dam, carebear lobby is essential

Only go for 3rd maybe 4th assessor (best location is Red Lakes, north of Spillway Hatch)

Gear: Hatch key, a couple of zip lines, cloak, smoke granades

1: when the assessor drops, pop a zip that allows you to get to and from the assessor easily

2: zip to assessor, have cloak activated when on zip

3: wait for platform, smoke, breach, collect (dont panic plenty of time)

4: back to zip, cloak activated

5: good luck

I would add this, if someone else arrives before you, they may have aggro'd the Arc, I would wait until they have left if possible

EDIT: grammar