Same problem is happening with me, I have a level 37 operative sniper, and I am getting help from a level 50 jugger, and we were only doing the class quests and it was working OK till this one. we cleared over 55 mercenaries and the loot didnt drop. We tried to abondon the party and let me do the first hit, but that time it didn't drop any loot for me because I didn't deal enough damage to the mob.
What I feel is that this is an "intented" bug that blocks people to rush their class quest at early levels, at least for imperial agents,and it hurts that it had to happen at the last planet's last quest. I cant kill the mobs because of near %98 chance of miss chance, and it doesn't count as my kill when my party kills it for me.
Sad panda is sad.