This is a common topic so I decided to try and help out by writing a FAQ.

Q. How do I collect souls?

A. To collect souls you will first need to collect Shrunken Heads found through out each level. After you collect a Shrunken Head you must activate your Jackanism Sword by pressing Left Trigger, Right Trigger and the "A" button, simultaneously. Then, when you attack enemies, you will get their soul.

Q. At what point will I start to see the Shrunken Heads that I need to collect?

A. You will start to see the Shrunken Heads in the second level(Port Royal)

Q. There is a Jackanism Chest in the first level, how will I open it if there aren't any Shrunken Heads in the first level?

A. After you finish a level you will be able be given the option to go back and retry from the "Extras" menu at the start of the game. If you save a game while you have three Shrunken Heads then when you load that save you will still have them, no matter what level you select.

Q. When I activate my Jackanism Sword there is a green explosion around me and it usually kills the enemies, do these kills go towards my soul count?

A. No, unfortunately not. If you are specifically trying to collect souls then try not to activate your Jackanism near the enemies as they will not count.

Q. Besides finding Shrunken Heads inside of chests, is there any other way to get them?

A. Yes. You will sometimes find them sitting in the open but more importantly you can get them by killing enemies with your finishing moves. This will prove to be invaluable when trying to get your 100 souls.

Q. How many Shrunken Heads do I have to collect to activate my Jackanism Sword?

A. You only NEED to collect one Shrunken Head in order to activate your Jackanism Sword. However, the more Shrunken Heads you collect, the longer it will last. 

Q. I activated my Jackanism Sword near the green chest but it did not open, why?

A. In order for you to open the chest you MUST collect three Shrunken Heads. It will not open with anything less than three.

Q. How many Shrunken Heads can I collect at one time and does it tell me how many I have?

A. The maximum amount of Shruken Heads that you can have at one time is 3 and you can see the amount you currently have in the upper right hand corner of your screen. One green dot above the picture of the Shrunken Head means you have collected one Shrunken Head, two green dots means two Heads collected but when you collect three Heads the eyes will light up in the picture of the Shrunken Head.

Q. Can I collect Shrunken Heads with anyone other than Jack?

A. No, Jack is the only one that can use the Jackanism Sword therefore he is the only one who can collect Shrunken Heads.

Q. What about the levels that I don't play as Jack, how will I open those Jackanism Chests?

A. Not to worry. If you don't play as Jack on a particular level than there will be no Jackanism Chest.

Q. To get 100 souls collected can I just repeat the same level over and over untill I reach 100?

A. Ok, here is the tricky part of collecting souls. The game will only take your highest soul count on each level. So, if you do {insert level here} once and collect 10 souls than you must collect more than 10 souls the next time for to you get credit.

I'll give you an example:

If you were {insert level here} earlier and lets say you collected 15 souls and you retry the level and get 14 more souls, you will not get credit for them because it only takes the higest amount of souls you get in in each level which was 15. 

Ok, now, same situation but on the retry you get 16 souls. Then you will only get credit for 1 soul because you already have collected 15 souls. 

So now here is the problem. There is no way to tell how many souls you already collected on a particular level so you will have to over estimate to play it safe. Ok, say you have 67 souls so you need 33 more and lets say you collected somewhere around 15 so far on {insert level here}. Round it up to 20 to play it safe and add that to the 33 you already need and there is your magic number of souls you need to collect. 53 souls on {insert level here}should get you the achievement, assuming that you only got somewhere around 15 on this level earlier. 

The above example was assuming that you will try to get your remaining souls on one particular level.

This thread is for open discussions so feel free to post any more questions that you have? - CD