I had a hard time finding a happy medium of not leaving the element right next to each the ghost while also not making the hero run across the whole level each time they absorbed the right element to take out a ghost. However, as you noted, you can just ignore the ghosts -- embarrassing secret, there are probably only one or two ghosts per level that have to be taken out for the player to advance. I thought about adding a score or a way the player can't advance until they neutralize all the ghosts, though I did not have experience with that and was completely tied up with level design. I think I will plan on adding a final score for each level based on how many ghosts you absorb.