You can find the lady who gives you this quest at the very top of Figaro Castle after you’ve broken out of the prison. She complains about monsters disrupting her tea deliveries and asks you to take care of them.
You are tasked with killing 5 Red Captains which can be found in the Watchplains near Cornelia. These particular enemies don’t spawn super often, so you may have to grind for a bit. You should also be WAY over leveled for them, so defeating them won’t be a problem.
Once you’ve hit your goal, return to her and she will give you an x-potion.