Only the heaviest of the heavy Promethean classes is a bullet sponge and that's because he's essentially a stationary Hunter/turret. The Prometheans otherwise have much lower health, instead focusing on damage output and cunning as well as teamwork to be a challenge. Watchers can provide some pretty aggravating bonuses if not dispatched properly, but have their power sets so separated by class that it's more a matter of picking your battles wisely than throwing your controller out the window.
Not including larger sentinel-like minibosses (which, upon "death," could be used as vehicles), my Promethean listing would be something like:
Pawns. Weakest enemies.
Crawlers, at least three classes - an Alpha, a Beta, and a Revival class. Alphas are slightly more heavily-armored and lead packs, Betas are your bread-and-butter enemies. Revival units are pawns trying to make it to "the end of the board;" they're weak but incredibly agile and make beelines for sites of dead Knights. If they reach a digital corpse, they'll sacrifice themselves to bring it back.
Rooks. Between Crawlers and Knights in power.
Sentinel Minors use the Oscillator's primary fire. Sentinel Majors use the Oscillator's alt fire (a weaker Sentinel Beam that's deadly against unshielded targets, meaning you want to take these out immediately if you're caught shieldless). Sentinel Primes use the Sentinel Beam which is about as powerful as the SAW now, and are shielded.
Bishops. Support units incapable of damage.
Watchers come in a few varieties now and can't revive anything. They project different-colored beams onto targets so you know what bonus they provide. These include Damage Boost, Speed Boost, Overshield, and Active Camo. There are also the asshole Watcher Spotters, which will constantly try to stay near you emitting a loud noise (but doing no damage) to give your position away. If you kill a Watcher's Knight but leave the Watcher alive, it will work for you until killed or replaced.
Knights. Second-strongest units.
Knights come in a few varieties.
Lancers are jacks-of-all-trades specializing in Suppressors or Light Rifles.
Hackers will run around the battlefield composing equipment, disabling vehicles, and so on. They WILL attempt to shred you to bits if you get in their way, but clever movement can outsmart them into getting assassinated.
Snipers are the weakest Knights and will use either Light Rifles or dual Beam Rifles. They're different from Jackal Snipers because they're constantly finding new vantage points and can hang from surfaces like trees.
Grenadiers will spam the shit out of light explosive weapons and use unlimited grenades. They aren't meant to kill you so much as they are to disorient the shit out of you.
Heavies look similar to the original design for the Knights. They are the ONLY bullet sponge enemies, but are huge targets and barely move. They're more like mobile assault platforms similar to Hunters, but with more damage output and less Melee capability. They use two Nova Cannons (Promethean heavy weapons) mounted on their back; damaging them enough would cause them to drop one Cannon, which you could use to kill them much easier.
Queens.
Supplementary characters that don't engage in combat, instead aiding you or guiding you through levels. See Engineers or "Ambient Promethean Knights."
Kings. Mini-Bosses
Kings are "Relics," which are essentially legacy Promethean soldiers that survived in powered-up robot bodies, most likely awakened by Chief at some point. You'd generally fight them in one-on-one encounters save for a few Watchers or Sentinels. They'd have special combat roles such as relying on space martial arts or Jetpacking to spice encounters up a bit, but would likely only show up once or twice per mission if that.