Kee..easy method on Isla Cruces level...
As Jack, run to the side and climb up the
rope and use the lever to open up the cave at the corner. Switch to either
Will or Elizabeth and go into the cave and stay there until it drops so
they get trapped in the cave. Control Jack again and go up the rope once
more and open the cave. Quickly control whoever you put in the cave for
a second and move them to the back of the cave, then switch to the
remaining character and move them toward the cave. Hopefully you can get
both in there in time. Switch between them quickly if they come out,
and hopefully you can lock both in there.
Then as Jack, the fastest way killing an enemy is the
AXA combo. Do this until you can do a special move, and do it. Hopefully
you get a skull (I say there is a 10% chance of it occurring). When you do,
move out of the way, but not too far, and activate it. DO NOT ACTIVATE IT
NEAR A GRENADE otherwise it'll explode, and probably kill you. Now harvest
a few souls. If you are lucky, you'll have a few enemies near you, and with
one skull you can get a maximum of five, maybe six souls each time.
With this method alone, you could get all the souls within an hour, an hour
and a half (excluding the time it takes to get to this point of the level).
It's boring, but just keep at and stay away from grenades at all times.
Once you are done, or had enough, kill the leader by the cave to finish the