Sorry, I don't know exactly which one I missed. I think it was somewhere halfway through.
Regarding the trial and error, maybe its more that the limited moves (oxygen) conflict a bit with the puzzle aspect of trying to find the position of the ghosts. I'm using the lantern and flashlight to explore the level, but when I move around too much I have to rush to the exit and will probably die, thus having to redo the level with more knowledge. This means I'm just going through the motions, marking ghosts that I already know the position of, instead of deducing where they are. Hopefully this explains it better.
To be clear I think the levels are great and enjoyed the game a lot!