I removed these examples because That One Boss is about a specific boss or two, not just hard bosses. If every boss is hard then it's likely Easy Levels, Hard Bosses instead.
That One Boss: Crashers are this for the unaware. What makes them so difficult is the fact they very rarely come alone and by the time they die they can be midway to the exit carrying three cores and leaving them as easy pickings for smaller, more fragile enemies. The worst part about Crashers? Later on they come in packs.
Turtles are stupidly tough for their speed (not as tough as Crashers but far faster) and the aliens they carry inside them often cause as much trouble as they do. On later levels they can bring Crashers and Rumblers.
Rumblers. Racers on steroids that are as tough as high grade Rhinos, without Temporal towers it's not uncommon for these things to just blaze through everything, pick cores and leave before the player even has a chance to built another tower.
Seekers later on becomes this. They can start summoning things like Racers and even high grade Bulwarks.
Juggernauts are just like Crashers, but with shields, although they are very slow they take forever to bring down even with Level 3 Towers.
It's quite telling how prepared you need to be when pretty much every boss enemy is listed here.
I removed these examples because That One Boss is about a specific boss or two, not just hard bosses. If every boss is hard then it's likely Easy Levels, Hard Bosses instead.