If you are talking about the Power of Hoodoo Quest, check the winning condition. It should say something like Defeat Old Scratch. This means that you should attack only Scratch and, if you concentrate on him only, you shouldn't have too much problem if you've been doing all the Quests up to that point. The only time I would attack someone other than Scratch is if I couldn't attack Old Scratch. However, don't continue the attack on anyone other than Old Scratch.
I agree with the above poster. The key lies in the victory conditions. You only have to beat Old Scratch.
My winning strategy is to wait and let the enemy advance on my position. Usually Old Scratch will charge out on his own and you can swarm him on the next round. If he isn't close enough by the second round, wait another turn.
Since I am a Swashbuckler, I usually prep myself on Round 1 by using Walk In Shadows on myself so that I can use my Sneak Attack the next round. Buccanners will probably prep with Kraken's Coils so that they can use Mighty Charge, and Privateers / Witchdoctors will use their best buffs.
When he is close, focus all of your effort on Scratch and have everyone use their best powers.
Your Bucs / Privateers should Crit Strike and Bonnie Anne should open fire directly on Old Scratch so that she can get in an additional Burst Fire attack (if that is what you picked as one of her epics when you promoted her at 8th level).
It has taken my Swashbuckler no more than two or three rounds to take Scratch down and I have seen Witchdoctors take that long also. Privateers may take an addtional round or two to win since their damage output is somewhat lacking, but they can better optimize the damage output of their companions.