The Immortal Master
Roshi has lived over 300 years whereas Genkai lived a fairly typical human lifespan. In those centuries of living he's become so ludicrously skilled that he's known as the god of martial arts and the invincible master. Even the skills of a 30th level black belt (in the real world 10th level is usually the highest level reserved for grand masters) were absolutely nothing to him. That black belt also happened to be a werewolf who trained by punching trees down, something Roshi actually hasn't shown the striking strength to do. Genkai's physical advantages won't help her against a skill gap this huge.
Should Roshi's fists fail him he has other unorthodox ways of winning a fight. He has the iconic and simple kamehameha for when he needs unstoppable power. When this move was first shown it accidentally blew away a whole mountain at max power. Roshi has been able to catch and redirect a kamehameha from Tien with his bare hands before which shows that he can block a lot of power.
The Bankoku Bikkuri-Shou which I'm only going to type once is an electric attack that holds an opponent in the air while causing them excruciating pain. Goku escaped it only by transforming into a great ape and even in his oozaru form the attack held him for a while and continued causing him pain. It was clearly a last resort and Roshi wasn't entirely sure that it wouldn't kill Goku. Although this attack is devastating it also prevents Roshi from doing anything for the duration of the attack.
Roshi is fine with using dirty tricks to win, and his dirtiest trick of all is hypnosis. He was able to put Goku to sleep with this technique and was also able to trick a werewolf into thinking Krillin's head was the moon. I don't know of any way Genkai could counter this.
I'm sure you think speed is Genkai's greatest advantage over Roshi, and it certainly seems that way at first glance. I'm sure you've seen the scan or scene in which Roshi and Krillin have a complicated exchange which is over before anyone can register that the fight has started. If not it goes something like this:
They come at one another, Roshi throws a kick, Krillin ducks under it and throws a punch at Roshi's groin, Roshi spits at him and makes him pull back, Roshi goes for a jab but Krillin snorts at him and makes him back off as well, they both actually stop and rub their chins while they think about what to do next, they agree on a game of rock paper scissors, Roshi wins and distracts Krillin by telling him to look at something off to the side, Krillin looks and gets kicked in the head by Roshi as he leaps into the air, Roshi does a superhero landing and Krillin flips through the air before landing on one foot and regaining his balance for a moment.
I don't really need to explain why that's so damn impressive. The conclusion of all this is that Roshi is too fast, too skilled, and probably won't even need his more unorthodox moves to win. He'll just make Genkai look like an amateur.
(@gearsecond659 I'm planning to add scans to this and maybe some more feats that I missed, but I figured I've made you wait long enough so here's the ugly rough draft. Once I've put the finishing touching on it tomorrow feel free to reply whenever!)