Since both of these combatants rely heavily on robot armies, I should take a moment to go over Umbra's Shadowbots. These are the main defense force for Umbra within his Shadow Fortress. There are two types of Shadowbots, a dog-like land version and a bird-like flying version. Both are pretty big as it took a dozen or so to cover Mighty Orbots. As Mighty Orbots measures to be about 150 feet tall, that would make a single Shadowbot about 18 feet tall. It appeared that in total there were easily over a hundred of them. Both are equipped with lasers and are susceptible to ice attacks.
Now on to Doc Terror. Doc's Doom Drone army is no slouch. Made up of four main borgs - The Strafers, the flying drones, the Traumatizers, the walking drones, a 6-wheeled tank drone, and a water-based drone - he's got more than enough firepower to match Umbra's. While the last two drones don't get a chance to show off their abailities besides laser cannons, the first two do. Strafers are capable of space flight, and the Traumatizers have a torso-mounted missile launcher. Traumatizers are said to be two tons, bu are also easily beaten; three of them were defeated by just throwing rocks at them.
In addition to his Drone's, Doc has a portable cyborg maker, capable of turning any living thing into a cyborg. He does it in three episodes I watched, making various borgs out of fish, dinosaurs, and lastly, a gang of punks. Yes, they're all extremely stupid looking, but it's very fast. He was able to make his fish bog army in a matter of hours. The borg maker can be moved from base to base and he has it stored on his big green spaceship, which is also capable of space travel. So he'd have no trouble making it to the Shadow Star.
Now let's look at Doc Terror himself. Being half-robot comes with a lot of perks, namely weaponry built into his robot arm. Normally it is a open barrel blaster, which also houses a strong electromagnet. His strength is also above normal, as he's able to break a desk in half and knock down a metal door with it. The blaster can also be fitted with a capture claw and a missile launcher. Unfortunately, since he has a henchman in Hacker, he generally stays at his base and doesn't engage the Centurions himself.