Voting Marauder
From what I know of Undyne it seems that she is well trained in her role and in combat however marauders were previously night sentinels which come from a race of people who live to fight so at the very least, skill wont be a significant factor.
Marauder
Marauder has a clear mobility advantage, shown through his agility in the game where he is able to do quick side steps and swiftly jump up and play surfaces on top of the marauder's ability to double jump.
The Marauder has the defensive advantage, his durability being higher than hers due to scaling far above his value as his profile suggests but also his shield being able to no sell attacks that injure him, being capable of pulling out a shield nigh instantly.
Marauder also had he offense advantage as not only does he have his ap advantage but his Argent Energy sword has heat hax that far surpasses Undyne's resistance as it is at least equal to the plasma rifle which is stated to liquify enemies (said enemies being able to bathe in lava) to 2 megakelvin laser heat and at the highest end, heat that is higher than planck temperature. Basically, if he hits her she is not blocking it without recieving grievous damage.
Additionally, Marauder has his wolf, although it won't be much trouble for Undyne to deactivate it as it only requires one hit to do so, it will still be able to add additional pressure.
Plus Marauder has his shotgun and energy beams (said beams scaling to his axe heat) for offensive versatility as he can now deal with her at close range and long range.
In terms of range they both have tens of meters so they are equal in this department.
Despite this, Undyne still can apply pressure to the Marauder, her regeneration will allow her to regenerate from some of the initial attacks from Marauders, Shotgun blast or a horizontal bisecting swipe of his axe, however sometimes the Marauder does a vertical swipe which should kill due to it exceeding her regeneration's limits.
She has to be careful though as the Marauder should have the battle iq to go for more severe attacks once realising that she can regen from ones that won't work so this is likely only something she can rely on once if she gets lucky.
The thing that makes this battle more competitive is due to two factors, Undyne's Dura negation and her yellow spears.
Her attacks target the soul meaning that it will one shot Marauder as he has no soul resistance, unlike Undyne, the Marauder can't rely on getting lucky with getting hit by an attack he can regen from and so her attacks are technically more dangerous (his shield still acts as an effective defensive measure though).
Plus her yellow spears that are made to counter shield wielding opponents by going behind the opponent at the last second. In game, Marauder can be affected by things that hit him behind his shield such as throwing a grenade behind him while his shield is active so it is very possible the Marauder could get killed this way.
Although the fight could go either way due to either character being able to one shot the other (Undyne more reliably than the Marauder), the Marauder's greater mobility and Undyne's tendancy to not use yellow spears right at the start makes me think Marauder wins more times than Undyne as it only takes him one (or two after seeing her regenerate) attacks to kill her.
