San's ATK is 1. As far as I know, Sans cannot be fought unless Level 19 is achieved, at which point, the HP of Frisk/Chara will be 92.
Sans is also explicitly described as the weakest monster in the Underground, & IIRC, don't all the other monsters have Durability Negation due to using Soul Attacks as well?
And yes, Frisk resists Soul Manipulation, but Sans is also unique in that his attacks inflict damage regardless of Frisk's DEF or Armor, unlike any other Underground enemy.
So given that he uses Soul Manipulation AND his attacks ignore DEF & any boosts to it & invincibility frames, Sans negates durability to a greater extent than any other monster in the Underground.
But he's still very explicitly weaker than all those other monsters, & his stats show this.
Yet nobody even comes CLOSE to one-shotting Frisk, Sans least of all. Sans needs about 92 hits to take out Frisk when he fights him.
So WHY would you argue that Sans should be doing more damage to someone else, based on Frisk's Resistance to Soul Manipulation when?:
1. Sans cannot OHKO him. To KO Frisk, he has to hit Frisk nearly 100 times. This is less easy to achieve against an opponent in a speed equalized match, especially someone like Marx who flies, teleports, has body control & can make Black Holes.
2. Sans, more than anyone else in the Underground, ignores Frisk's resistance, seeing as he always hits for the same amount, no matter Frisk's DEF & any invincibility they should have?
Also, Sans's usual defenses don't really help him.
>Marx attacks
>Sans goes lol and dodges to the side (As he canonically does in the battle) or teleports
>Sans still dies because Marx's range, because Marx's range is "Standard melee range physically, interstellar with projectiles and shockwaves (Scaling from Kirby)"
If Marx makes say, an explosion with his energy manipulation that's omnidirectional, & Sans's teleportation is instant, he'll almost certainly end up in the explosion's range.
Even the explosion or its blastwave or whatever is just dying out as Sans reappears, even a TINY fraction of 4-A power would be well in excess of what's needed to thoroughly kill Sans. Likewise, a black hole with that range would be similarly devastating to Sans.
Sans's teleportation range might be listed as Unknown, but I'd be skeptical that it's Interstellar range. Even if it was, him expecting to need to teleport that far before seeing Marx's first attack (The AoE of which could easily kill him.) is unlikely.
And on the offensive side, what stops Marx just teleporting away from Sans's attacks, especially in Speed Equalized.
Any non-physical attack from Marx has the range and power to kill Sans, teleportation or not, & Sans needed 92 hits to kill Frisk (Who could still dodge, may have had the turn system delaying them, & Frisk might heal, meaning more than 92 hits could've been needed for Frisk) in spite of being more durability negating than other monsters.
Given that Sans's AP is explicitly the weakest of any Undertale monster, & he still needed so many blows in spite of all his durability negation, I don't think he would kill Marx fast enough before Sans gets one shot, assuming Marx doesn't dodge/teleport in this speed equalized match.
I'm voting Marx, Medium-High Difficulty.
Worst case for Marx, both sides fail to avoid one anothers' attacks (Because Sans has Danmaku & Marx has interstellar non-physical attack range & black holes.) & they both die for an inconclusive.