Not technically "invincible" even if Mario is utterly impenetrable, he still has limitations.
There are holes, being crushed, running out of time, beating a level, Tap-taps, Haruhi, Mokou, Kaguya, Non-living things, bodhisattvas, Palkia, Dialga, Arceus, Homura, and Getting stuck in bad level designs.
Why? Because being a mere palette swap of an already existing costume dose not make you a super deity.
EDIT:The same gose for the Super AND Hyper Sonic forms.
-------------------- Perfection is something that requires something that incompletes it. Perhaps it also breaks itself.
The current avatar I have is from Neslug from DeviantART.
So far I have to say the best answer is whoever suggested Hotlips from YI. In this fight, only natural abilities or characterizing tools (like, say, it's not like Link couldn't go in with his Master Sword) are allowed. Hotlips can spit lava naturally, which can kill WTM. That idea is ingenious, so I have to say it's the best. (Maybe a Blargg would also work in that sense- they're made out of lava.)
Kirby- Not sure. Even if WTM was inhaled Kirby would still touch him. He would explode from the inside or something like that.
Goku, Broly, and other DBZ characters may stand a chance. They have some pretty powerful attacks, to say the least. But still, I don't know if anything could penetrate permanent invincibility. It could be a possibility.
I don't know about freezing either. He'd be frozen but not dead. He could probably use his tanooki spin to break the ice away and then go in for the kill before the fighter can use his ice again.
Minecraft Player/Steve, put simply, would not stand a chance. Lava bucket may work, but remember, no tools. Only his characterizing weapon, which in this case, is his pickaxe. That's all he would be allowed. /kill would probably cause a universe-destroying paradox if used on WTM.
So, so far, Hotlips is the winner. Can anyone top that?
-------------------- tired: 'JOYCON BOYZ'
wired: 'The Fellowship of the Switch'