Cirno is an example for an enemy where you can use the fact that he runs to you, as he doesn't shoot any bullets directly at you. If you walk on top of the one way platforms in the beginning and wait for Cirno to get under you and jump, you can drop down on him without getting hit.
Thank you for a useful bug description, I'll have a look at that later.
The bullets stop once out of view as a result of the enemies being hacked together as an afterthought for the jam. They are not integrated with the map editor, they are just spawned at hardcoded locations and do their thing, instead of having dummy enemies replaced with the real thing when you enter a room. The easiest way to approximate that was to skip everything that is not in the view. As all the piled up bullets are flying away from you when you transition to another room it shouldn't lead to any unfair situations.