The last flesheater isn't a problem at all to avoid. Just make sure you have a timer of 30 seconds running. That's about how long you have from that he leaves the balcony to when he enters the balcony again. Start it when he leaves the balcony, if you're not up within 20 seconds (~3 failed tries), just go back to the stairs area, make sure you're actually in the staircase room, and wait for him to leave again.
Wash, rinse and repeat. No need to use another invis pot there.
Has anyone tried to do this on PTR? Does it still work post-2.1?