If it's a solo duty quest, where you enter in without party, it's made so that you're supposed to clear it without using potions on someone else. This means you'll most likely be in the tank's role taking all enemies on yourself so they won't kill this friend.

What I would also suggest is that you'll upgrade your gear so that you can kill enemies fast enough so that no one dies. Secondly I suggest you level marauder to level 15 and unlock Dragoon job as that's the advanced version of Lancer. To unlock Dragoon (DRG) you have to have done mainstory line far enough, up to Sylph-Management quest and finish it, then talk to Ywain (Lancer's Guild's master) in Old Gridania. In terms of having done the Lancer job quests from that npc as well. I hope these help.