Demonic Spiders: The regularly spawned crawlers are effortless to defeat on Easy and still easy enough on Normal. On Hard, however? They can sponge way too much damage before going down, so it is almost impossible to engage most of the Crab's attacks without a crawler shooting at you, making your airspace that much more deadly. And some attacks, like the Rambo attack, are pretty much unavoidable death if a crawler's also shooting at you, so the only solution is to wear down a previously spawned one and kill it just as this attack is starting to engage. And if that wasn't bad enough, they can also cause a Kaizo Trap by killing you while the cutscene after killing the King Crab plays!
That One Attack: The spinning pearl laser. It's difficult to keep up with, but the true evilness of the attack lies in the fact that the predetermined trajectory you must take to avoid it will often put you into the path of a random electric eel that will take a chunk of your health off or, worse, slow you down just enough for a laser to mow you down. And if you still have a crawler minion to deal with (as you most likely will on hard mode), that attack will be nigh-undodgeable.
The Rambo attack can also be this, again due to the crawler minions, because the safe spots are so small that moving into one will make you easy prey for a crawler's lasers.
is probably the most epic thing you'll ever hear in a flash game.