This quest has bugged a lot in the past for my girlfriend and myself. I just did it last night on my Sent and it would not work at all. Then I logged to try again. Logged back in, was attacked by a conquest droid right away, I smacked it, my girlfriend killed it, we were not in a party, and that one counted. We joined back up in a party, none counted. I left the party, tagged one, then she helped me kill it, it counted. So did the next one. So, the quest is very buggy, for me it seems that it did not want me in a party doing it.
When my girlfriend did it, we were both in a party the whole time, only 1 out of every 4 or so seemed to count though. I recall it being very very random. So you are not alone. Just keep trying it, if you haven't already finished it, it should eventually take.