Can you concentrate on the one doing the healing? I didn't have too much trouble with those guys (an exploding arrow for the group, followed by point blank splitting arrows for whoever came closest (or I went to them), and normal/poison/stun arrows when those were on cooldown).
Maybe run away a bit, and see if they spread out coming after you, so you can focus on one of them.
Use any boost potions or skills you have, to see if that will give you an edge. Check if you have jewellery with a summon skill bonus, to add another ally to your creature (even if they don't do a lot of direct damage they may interfere with the Black Ring soldiers' ability to heal, or at least provide a distraction), or can Charm one of them, use Rush Attack then Firewall to help with a melee attack, etc.
