system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel] How much damage can Santa Claus cause if he really wanted to?

He's an Omega level mutant, among the strongest.

If he really wanted to do bad, how much bad could he do and who can stop him?
gpt
The proposition of the question is flawed. Santa will not break bad. He frequently bails the heroes out. 

BUT... in the effort to make a good faith answer: 

Santa turned the Brotherhood of EVIL Mutants into inanimate toys, and has brainwashed dozens multiple times to keep his existence secret. His potential for destruction is on par with Baron Mordo or another high level magic user. 

So, who could stop him? A short list of a list that is possibly much longer: 
Doom. Magneto. Dr. Strange. Invisible Woman. Probably Black Knight or anyone else with an iron weapon. Squirrel Girl. 

While Santa Clause registers as an Omega level mutant, he’s really a high level magic user in the 616. Let's explain the choices: 

Doom is both a master contingency planner and mixes magic and tech to superior effect. He already has a device to cap Santa Claus. 

Magneto can mess with any metal he has on him, and fashion iron missiles to take him out (iron being magic resistant in 616). 

Strange or other Sorcerers Supreme are used to battling supremely powerful dark magic, like Dormammu, so would be ready for Santa. 

IW can deprive him of oxygen, which could end pretty much any fight.

Black Knight's blade makes him immune to magic, and he's an excellent swordsman. 

Squirrel Girl is Squirrel Girl... 'Nuff Said.