First of all, I suggest using the Paladin armor - the Unwavering passive makes the monster hit against your light resistance. The shield from Paladin Armaments has a similar gimmick, iirc, so you always take damage against your lowest resistance, whether Ringleader uses darkness or fire.
The easiest way to kill him is to destroy a single skeleton - this removes Freedom from the monster. Then use your Love Potion to paralyze him for the next however many rounds you need.
A less cheesy strat is to use Paladin in FD mode, with Panic toggled on, and cast Resplendent Surge, keeping the monster's damage and accuracy low. Attack the skeleton, kill it when Ringleader starts drawing power from it, then repeat with the second skeleton. Killing them both removes Bone Wall status from Ringleader, lowering his defences and hp, leaving him fairly easy to kill with level 19 and 20 skills.
P.S. It might also help to charge Defiance against the quest's mobs, just in case.
< Message edited by Primate Murder -- 10/13/2020 5:51:22 >