Geralt doesn't exclusively wear metal armor, and he would know
exactly what kind of threat he's facing by taking on Magneto. To Geralt, Magneto is the equivalent to an epic-level Terramancer.
I think it's fair to believe that, given his connections, Geralt would be aware of the fact that anything magnetic would be dangerous to wear or utilize in this match.
As far as armor, Ofieri scale armor contains no metal and wearing it intensifies Geralt's abilities with magic. Another non-metal armor item I'm sure he would also opt to wear is Toussaint Ducal Guard Captain's Trousers, which grant him increased resistance to piercing, bludgeoning and slashing damage on top of an extra 400 points of vitality. Both armors are made of leather and gem dust.
Certain other armors are made of made up of metallic components that were invented for the game, which I'm willing to believe are vulnerable to magnetism in that there's no data on the subject available.
As far as swords go.
Geralt would need to disrobe and rely entirely on his signs(powers) because he is fresh the fuck out of luck with his swords.
*facepalm*
Norcal, Dear. There's a method in place to test if you possess genuine silver, and it's called the "Magnet Test". If a magnet sticks, you know it's
not genuine silver. Magneto has been stopped by sentinels made of non-ferrous metals, thus he's not able to control anything that qualifies as metal.
Geralt would need to ditch his steel sword, which is a moot point because Geralt wouldn't use a steel sword on an opponent who's obviously a mutant like himself.
Geralt would have to ditch his crossbow option, in that they all contain some type of iron ore.
I'm going with Geralt on this one, mainly because he has the ability to use Axii (mind control) magic. He's used this ability to tip fights in his favor when fighting warriors of The Wild Hunt, these individuals wear armor from head to toe. Magneto's helmet isn't going to stop Geralt from giving him a massive brain dump in the middle of the battle.
Magneto can't fly because after being influenced by the Axii sign he's not sure of what year this is, and is annihilated by any pick of Geralt's silver sword options.