This is something I wish they would explore more. Marc does not seem to be investing in ANY capability to submit anyone, or restrain them with any type of wonder-tech zip-ties, or enhance himself with tungsten brass knuckles, or explore other options for these viltrumites.
Powerplex was his first instance of needing to show true restraint and compassion while subduing someone simply with submission/restraints. The problem is (and this is why I liked that episode while others might have not) Mark failed in multiple key moments that shows he still needs to grow:
1) He ignored the first time Powerplex asked for Invincible. This is that spiderman 2 moment of giving up the suit and thinking 'someone else will do it'.
2) He should have taken their initial battle outside of that building. The building itself was not going to collapse or anything. There was no reason to keep the battle inside. This points to arrogance.
3) He literally almost convinced Powerplex through his words instead of actions, but he made the mistake of talking from a 'superior' point of view by lecturing him instead of trying to see his perspective by using his own father as a mutual talking point. This points to a subtle superiority complex.
I will say that the only way I can see him ever subduing a full-on viltrumite would literally be if he met them, solo, in the future, because the future Immortal talked Mark allegedly gaining a crapton of muscle and power after almost succumbing to a virus. Right now, I can understand why Mark would rather just kill a Viltrumite than take that chance. He isn't strong enough yet to look at them like equals or as if they are beneath him.
What happened to all those pulp adventure books his dad wrote that all the clues to defeating viltrumites?
Allen the Alien took them to the Viltrumite resistance force from what I last remember, but I could be wrong on that timeline if anyone wants to correct me.
What is the 'human' contribution and should we even care, or just consider Marc to be effectively a full blooded viltrumite? Why are SO MANY other shades of Marc outright homicidal psychopaths?
From what I'm gathering (and this is only a theory based off of everything I have seen in this show), Viltrumites might not have an adrenaline factor like humans or they might have some sort of limiting factor to their strength. It's the only explanation I can think of for Mark slightly increasing in maximum strength and getting bursts of energy during a battle, even when he went all out with Conquest, but Conquest not doing the same when he was met with dire circumstances.
I can't even remember why we should be against viltrumite "colonization" in the first place. I know they occasionally destroy planets but otherwise what hardships do they bring versus a reprive of the all too frequent random rampaging monster that presumably an 'on station' viltrumnite would dispatch quickly. I feel like if they could show what the typical viltrumite run planet looks like and put it to a global vote, most normals would vote YES.
As shown last season through other dimensions, and explained by other Mark variants this season, the Viltrumites would enslave all of humanity, put them in camps, and execute people seemingly at random either based on population size (aka genocide) or for other reasons. This is why if you noticed, while Angstrom did say last season that nearly every Invincible ended up evil (except our Invincible Prime), he didn't quite state that every Invincible sided with the Viltrumites.
The other Marks hinted at this with some of their lines and we will hopefully see more hints of this in a future season as I was spoiled a bit on other Mark universes, as some of the Marks either went batshit insane and started killing anyone and everything, or they killed Nolan, or they fought against viltrumites
with Nolan, or they conquered earth solo and are it's sole owner/protector. Two of those Marks we saw were very, very powerful based off of what they claimed and again some spoilers I've seen, so it is the best possible option that they were quickly removed from the prime universe.