So this actually seems decently close, most of GK Markus' anti-undead and daemon shit won't work because Witcher vampires aren't ACTUALLY undead, same with Detlaff's mind manip as well as his sleep manip, and their base AP is nearly even. They should have comparable skill levels, their tankiness is relatively even too, Detlaff obviously has the superior regen, but Markus de Mandelot has not a bad form of regen himself, and can stack it with a secondary regen to double it's effects, has damage reduction that forces him to only actually take half the damage he should have taken, his shield can block most basic attacks and his aura gives him a chance to complete negate an attack, and has some really good healing items if he is pushed hard enough. With speed unequal, they are comparable in reaction speed, Markus has overall massively superior movement speed but that can be equalized through Detlaff's flight.
Kruber would have the over all superior damage output, via bombs (which can badly harm characters of comparable durability) and his conjured Holy sword can nearly oneshot if it lands. Detlaff has the range advantage with his bats, but they are also decently easy to dodge or outright kill entire clouds of them with a well placed incendiary bomb. Markus also has the superior LS which will pose a huge problem for Detlaff if he tries to grapple. Detlaff's intangibility is countered with Kruber's NPI being able to hit intangible and ethereal enemies, and while Invisibility would pose a tricky issue, Kruber can sense attacks coming from his blind spots and quickly parry, block, dodge or even counterattack. Kruber also has 3 potions that will GREATLY help in this fight as well, one that doubles his AP, one that doubles his speed, and one that massively reduces the cooldown of his Holysword so that he can summon it at a far greater frequency, he can possibly also gain extra bonus' from The Lady if he acts if she thinks he earns them during the fight, like perma-buffs to speed, strength, damage reduction, regen and cooldown reduction and while they wouldn't be especially huge buffs, every little bit helps.
Over all this is going to be a pretty huge brawl with both pulling out all the stops in their attempts to end the other, but overall, i think Markus de Mandelot just has over all the more consistent damage output and his amps and mix of damage reduction and healing give him the far greater chance to incap Detlaff via dismemberment/Lifting Strength
