I was having some issues with this event, in that sometimes it would burn down and take no time at all, and others bard would get overwhelmed and not get rid of all the pesky aliens fast enough. So now i just turn burns off, and leave them to it, make a coffee and its all done. The lua script is brilliant for this and Aten (for the garg achievement). but Shei health locks at 2% and the aliens can pile up when burning. Not failed it once since i turned burns off. takes a couple of mins longer but no issue.
One thing I have noticed though with this mission, is you can not get the achievements unless character is in zone when Shei dies, where as the others you can just TA in at the end and still get achievements.