•Some people were complaining that King Sombra didn't do much. The thing is, he DID, one thousand years ago. Unlike Discord and Nightmare Moon, he had no magical relic to deal with that would be capable of stopping him, because he already did a pretty dang thorough job of it a thousand years ago. He emotionally broke the Crystal Ponies into near depression (meaning they COULDN'T power the Crystal Heart unless they were cheered up) and hid it in his castle in a way that could only be accessed by someone capable of using his magic, protected by a door that not only moved by itself but trapped whoever entered it in their worse fear. And on top of that, finding it would allow him to trigger a trap that not only traps the one going after it but would let him spread his corruption inside the castle without him even having to get through. And the only one keeping him out anyway was slowly weakening and would eventually give out. Whereas the other villains needed to disable their Achilles Heel themselves, Sombra already did. And because of it, he only lost because Spike decided to tag along with Twilight. There was no need for him to actively attack and try to make Cadance's spell give out sooner, because no matter what the heroes did, it'd ultimately benefit him.
◦The layers of traps are even more impressive when you realize that all the Crystal ponies are Earth ponies, and getting by his traps all but requires the use of magic. A non-unicorn couldn't get through the passageway in the first place, and if they did they'd have to be able to get through the enchanted door (which would delay most ponies long enough for Sombra to notice the intrusion and lay down some whoopass), or up the stairs. And even then the heart is warded against magic anyway. He basically had the foresight to hide the heart in a location that none of the Crystal ponies could get to, and then cover the thing in spells that the most powerful unicorn in Equestria couldn't figure her way out of. And that's just the stuff that's still working a thousand years later; who knows how much security he'd have had up if he were still actually living in the castle.
Furthermore, the traps are deliberately designed to prevent unpowered Earth Ponies from bypassing them, as they require magic and flight.