I'm trying again now and I still am not sure on how to use the bow for the eye. The spots I would think to try are never valid targets (I'm guessing diagonals mess with "second row" calculation).
I think the bow would be better if it revealed walls/floors at very least, perhaps some alternate type of highlight (miss?) that doesn't count as revealed. I'm assuming it can't take out all enemies, but no idea where that line is at (lvl 8 or lvl 5?).
Other note (re: Is the Game Always Solvable): I've noticed it's possible for walls to cross the map, and with diagonal gaps not always crossable... if you use the available hammers on banners (like before the entire wall is revealed) you can get stuck that way.