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.
Goddamned Bats: Any and all of the enemies. Especially any shooters. And those huge swarms of zombies that try to encircle you. And those small robots that impale you with the spike on their head who are dang impossible to catch. And those fire-breathing robots. Yeah, let's just say all of them.
Zomboss' grab deals immense damage and is hard to avoid. Thankfully, it has short range, so you can tickle him from a distance with your weapon and never get grabbed.
Zomg's whirlwind attack covers a massive area and if you get caught in it, it can hit twice. Thankfully it has a ridiculously long startup time during which you can (hopefully) run out of range, and a recovery period after it during which you can unleash hell on him.
Smileborg has his blades that don't knock you down when they hit, allowing him to deal immense damage if you get caught in the attack. Be ever watchful and prepared to move away. The same Smileborg can summon Action Bombs, which is more of an annoyance than a threat because he's invulnerable while they're alive.
That One Boss: All three have potential for this (especially in Real Man mode).