You want an even better explanation? Most Viltrumites have had more experience punching down and holding back since they have had their full power for much longer while Mark hs had multiple fights in the series where he had to punch upwards or was weaker so he needed to use more combat skill to win. So if you’re only opponents for the last 1000 years has been creatures you’re too strong for you’ll start struggling against a person on your level.
You want an even better explanation? Most Viltrumites have had more experience punching down and holding back since they have had their full power for much longer while Mark hs had multiple fights in the series where he had to punch upwards or was weaker so he needed to use more combat skill to win. So if you’re only opponents for the last 1000 years has been creatures you’re too strong for you’ll start struggling against a person on your level.