These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: Rocket-launching enemies. They have at least 20 health, and their rockets, while destructible, will destroy you in three hits. The easiest way to beat them is to use a bomb, but it's not a good idea to get near.
The flying enemies are a nuisance in general, especially those that fly in a wave pattern. You'll be swarmed all over by them in certain levels, and they start firing aimed shots at a rapid rate later on.
However, should you run out of bombs and are stuck in a hard place where you cannot reach the Mooks, these guys will be your only source of bombs.