Well I finally got through it just by leaving Gault to finish off each wave to go and stand in front of one of the next wave's containers and just hit basic attack while the droid came out.
I have to think it was random / luck though as none of the things that worked for others worked for me, and this didn't work on the first few tries. Probably the same principle that worked for others with AOE, just couldn't get the timing right on my marauder's (few) AOE abilities
Good luck to others.