Although this has been talked about a lot, the first time I tried it I couldn't get it.
I was having problems with the mini-boss (in Isla Cruces) always coming up to me when I activated the jackanism power and ended up killing him accidentally sometimes.
Also, shrunken heads weren't dropping for me as quickly as some people have claimed they drop.
So here is my advice -
1. Don't forget to get souls on your first play through. By the time I finished the game I had 70 souls easily, and believe me I wasn't even really trying but you do have to at least consider getting souls along the way. One of the best places for me was Davy Jones Locker.
2. Complete the game first, then go back to Isla Cruces. You have a much smaller and more attainable target to get. Do NOT go to Port Royal or any other place. I spent some time there and heads weren't dropping frequently enough.
3. DO NOT even BOTHER trying to get a lot of souls in Isla Cruces unless you have TWO shrunken heads drop BEFORE the mini-boss comes up while you are killing the regular bad guys in this level. The first time I tried it almost no heads were dropping. The 2nd time, after I had 2 heads drop on me before the mini-boss they were dropping like crazy. So make sure this is happening otherwise you'll be waiting a long time unneccesarily between heads
This way I had something like 12 souls BEFORE the mini-boss came up. Then I only needed another 13 souls and because the skulls were dropping so quickly I got them in a matter of MINUTES.
4. I have read tips about locking either Elisabeth or Will in a cage but I never even did this
5. I read also on xbox360a to get the mini-boss to fight one of the NPCs (the guy with the wooden eye or the other fat guy thats his friend). This I did use and was CRUCIAL since NPCs cant be harmed and will save you from having to help Will or Elisabeth all the time. Just select Will or Elis. and draw the mini boss to the NPC and then kill the guy the NPC is fighting with, then he will begin fighting the mini boss
Like I said using this strategy I see no need to waste an hour or more collecting souls in Isla Cruces. You'll be done in a few minutes literally.
Hope this is helpful.
Thanks
Completed games: 24