I found that using Jack works the best because for what ever reason most the the soldiers occupy themselves with attacking Will. Attack the Spaniard as close to the main mast as possible. Hit him three times with the quick attack. When you do the Spaniard will let out a sort of a spanish battle cry, sort of an "aye-aye-aye-aye". When he does dodge roll out of the way. He'll do his thousand hit spin and that will damage the mast. Do that about 7-8 time and that's it.

it took me forever to figure that out, but you need to make sure he is close to the main mast. when he hits the main mast, it makes him loose some of his life. he can only be defeated by himself. once all the plates fall of have jack burn it.