Yes, I killed only two mobs WITH buff cure
Unlike your mage I FS priest, and I have only one attack - holy light. As a result, I'm trying to get the quest for 4 days In many ways, cursing all that you can.
As much as I myself advocate to FS, the game just feels that it doesn't want you to be FS right now. I was going to suggest to: get at least Cure and Aspersio up' then 'run in, move a bit backward so they group up, cast Aspersio'd Magnus with the thief as primary target, then run away for the duration of the DoT, then finish off by what you think is necessary, but in that case at least 3 or 4 should have died, or even 5 if you group them up well enough. This takes advantage of the 10 seconds (?) immunity you get in a new map as long as you don't attack immediately.
However, kudos to you for making it to 50 with just Holy Light. Did you level with a party all the way? I originally planned to do that when I was trying to get 50, but I rarely had anyone to party with so I'm mostly by myself. But then when I do run into an occasional party-mate, I find that my style (tabbing around DoTs) actually has me killing faster than a DPS. And before I completely get off topic, I'd like to point out that the borderline between FS and Hybrid right now is too thin for any favorable or significant distinction, especially after getting the 20 bonus skill points from ML. Don't get me wrong, I'd reset skills in a heartbeat if a future patch changes that fact.
I guess you have no choice but to get more, more MATK or buffs that increase output, if you're going to wing it with Holy Light.