When escaping Castle Frankenstein on level 8, you will pass through familiar rooms. When you get to the room where you first encountered pygmy bats, jump through the hole in the wall in which you came through to get up there. As you jump down, you will notice a pool of spilled goo off to the side from one of the containers. If you look closely, a glowing lightning gun will be lying in it. Add it to your growing collection of superb weaponry.