Well, it depends how much stock you place in GL and Strange auto shields, and whether they can speedsteal Juggernaut. But even then, the ebst tey can hope for is a draw.
Any character that controls Time could win. Or Space.
Theoretically, Pip, with the Space gem should be able to make each step Infinitely distant from each other.
But then it goes on how much stock you place on reaction speeds.
Unless you do something like 'Fate enters the arena with shields already up', without autoshields, he'd be blitzed nigh simultaneously with the ringing of the bell.
No one would stop him. In this scenario, you would imagine that some voice from beyond said 1..2..3.. fight. It would be over the moment the word fight was spoken. Basing this opinion off of the OP, the Flash would be hitting with a mass equal to his speed, and end up displacing the equivalent mass in terms of metrics to the far reaches of a large sector of space. Matter can not be destroyed but it can be displaced, which would also include the character being hit with that amount of force. Conceptually it would be like blurring the lines of what is matter and what was energy. No mystic would be capable of making a spell in time to counter this amount of speed, nor would your average reality manipulator have the time necessary to counter. The Flash is the most powerful Herald out there with CIS off. I would dare to say that not even a Sky Father would be able to contend with a wielder that was fully tapping into the Speed Force. I mean how the hell would they be able to?
PIS is what you get when a character like Superboy Prime survives after getting into a fist fight with any user capable of becoming one with the Speed Force. It's no wonder that he didn't destroy the entire universe while on that level.
The Flash as hard as it is to admit, is the champ.
With what time? He would be hit with enough force to displace every planet and star in the universe. Think of the amount of speed he would be hit with. The idea is absolutely crazy. Genis' chances of winning would be lower than reversing the laws of reality. In other words with CIS off he would die the moment that voice said fight.
Someone once said that if an object made of pure iron the size of a football stadium hit a planet moving at the speed of light, that it would destroy our entire universe. The Flash would be moving multiple times that speed based on his top speed feats in comics. Which would also make him far larger in terms of mass than a planet, and with the Speed Force shielding him, he would be far sturdier than iron. Just something to think about.