This is an interesting fight. Both fighters have plenty of tools capable of one-shotting and heavily crippling each other. Besides, I like the idea behind it, too. However, considering their profiles, I will note something about Geralt, and how the way he fights will change how easily the fight might be.
Physique-wise, honestly, Geralt looks more impressive from his feats, but Sub-Zero is rated higher on his profile. I don't know why, but I'll guess that Sub-Zero has a certain, but not notable, edge in strength and durability, but even then, Geralt's potions and powers should suffice in diminishing the disadvantage, besides, it is not like the difference is massive either way. They are both surprisingly nimble and athletic, so I don't think any of them has a significant advantage, at least not in this scenario. Skill-wise, Geralt seems the better one of them in most regards, and by quite a significant margin, although again, Sub-Zero is no slouch and he has some stuff, most notably the absolute zero ice, that will most certainly catch Geralt by surprise. The arena itself is... Pretty neutral. You can argue it favouring Sub-Zero a little bit since he has more experience fighting people directly and in open areas, but it's less of being an advantage to Sub-Zero, and more not being favourable to Geralt. A more confined ambiance would be really bad for Sub-Zero out of the sheer terror Geralt could inflict with his pseudo-guerrila tactics.
In their powers, we see something interesting. Both have quite the arsenal of deadly stuff, and by himself, I'd say Geralt comes a bit short in power strength, and winning in versatility. Geralt has so many more tools at his disposal, but the ones that Sub-Zero have, are usually more lethal. The Ice Clones, like how Abstractions mentioned, would be a very good move from Sub-Zero's part, although I really doubt it'd be a one-shot move (I mean, guys, c'mon, when do the caracters ever use the 100% most efficient kill combo option? I can guarantee to you that it isn't very much in Sub-Zero's style freezing someone and just blasting them with AZ frost attacks). His AZ attacks can very well instantly kill Geralt if he isn't careful, which, to be fair, he will be. Geralt has plenty of tools; none as capable of insta-beating Sub-Zero as his AZ, but more than enough to keep him on his toes. The time-slowing sign is an example.
However... There comes Geralt's equipment, and the fun part for Sub-Zero ends. Geralt has so many options that Sub-Zero has little to no way of combating that it isn't fun. The pain-inducing bombs and the several other items in his arsenal would cripple Sub-Zero if Geralt went right to them. Which isn't unlikely; I don't think he would necessarily use them, but seeing how pragmatic he is, he could. If he uses them, to be honest, I don't see Sub-Zero doing much. Otherwise, the fight can get quite prolonged, and it would be awesome to see, with both having good chances to defeat each other, but I see Geralt taking this out of skill, tactical ability and a larger variety of powers.
Geralt should win with a lot of difficulties, but way easier if he uses all he has from the get-go.