Go to Tunis and pick out a week pirate to prey upon, then when he comes out bombard him and challenge him to a duel. Make sure you are well equipped with a B or better sword and armor, when in the duel have him at your mercy by keeping at a 50 plus or minus ten life points but don't kill him. Use weak one level attacks until he can survive a blow.
Keep that up for 10 attacks and the duel will end. Then challenge him again and again until you have your desired level. note this works best if you have a Barge ship and have sunk all other ships around so you can duel away.