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: Helicopters, they shoot a stream of shots up to your character and can be hard to avoid. They also have quite a bit of health. Not to mention that those bullets can break through any object you're using as a shield and still hit you...
Unless it's the Amiga version, where they can only shoot directly straight.
White-uniformed soldiers, they do a combat roll when you hit them, becoming invulnerable to attack. They take three hits to die. And they chuck grenades at you...
Goddamned Bats: The regular soldiers. They appear in mass numbers and fire randomly.
Good Bad Bugs: When the truck boss moves offscreen, he actually stops. You won't be able to shoot it, but your grenade explosions will damage it. So all you have to do is chuck grenades at the truck until it moves offscreen, then chuck grenades at the left.
Hard Levels, Easy Bosses: If you have an autofire emulator, you can destroy most of the bosses in seconds. The fact that the bosses are never accompanied by regular soldiers or vehicles also makes them easier to survive at times.