If you don't wanna risk an exacto knife, just put a screwdriver in the hole at the base of the torso and use it to gently pry it open. That's usually how I do it unless I think it's glued (as in the case of the BJ's 8 pack Storm Shadow and any of the Ninja Force Figures), then I pry with the screw driver and slide an exacto knife to work the seam slowly.

Also you could leave it in the feezer for a bit. That tends to make the glue brittle and will make your work easier.

You may hav already try this, but before using any cutting device, try tu move one of the arms (holding it by the upper half) as a lever to push the two sides of the torso appart.