Flash doesn't have the durability to beat Superman consistantly, he'd have to get pretty lucky and avoid getting hit for a very long time while trying to wear Supes down.
But he doesnt need the durability if he has the significant speed advantage. And we already know he can knock out superman level characters (white martian)
Flash's arsenal is massive by now. A single IMP KOed Zum, who is around Supes's level of durability. Flash has vibrated through Superman before, so doing something gruesome like removing his organs Indiana Jones style is always plausible. Flash has actually made speed force constructs before. Flash can stop time, Flash can take all of Superman's speed, rendering him completely immobile. Flash can time travel to when Kal arrived on Earth as a powerless baby. His options are many, he's godly these days.
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I'd question how significant the speed advantage is, Superman is incredibly quick himself, and he doesn't need to be as fast as the Flash to win, just fast enough to land a good hit on him. And I really doubt what was enough to put down a White Martian could finish off Superman (Wonder Woman took several IMPs from Zoom and kept getting up...).
If it's pc superman, then on the spot instantly he could come up with a anti speed force power that erases the speed force from all comic existence, but pc supes can still move at ludicrous speed because the laws of physics and comics don't apply to him.
And if it is PC supes at his angriest then say hello to..
"PC Superman pimpsmacks the Living Tribunal, Death, Galactus, Eternity, Exitar, and Thanos all in one quick swoop. Then he takes out all of the street-levelers in one speed blitz. Finally he takes out all the rest of the cosmic beings with a planet to the face"
and...
PC supes anti comic contium heat vision, or the retcon punch.
Wally reacts within picoseconds. He runs so fast that he transcends time altogether. He ran to the freaking end of time to escape Death. The speed advantage is absolutely massive. Zum's (the martian) durability is pretty close to Superman's.
__________________ DarkCrawler is my hero... RESPECT LEONARD NIMOY!!!
Nono, Pre-Crisis Superman will fly around them with speeds of MILLION times of lightspeed, REVERSING TIME, and SENDING THEM TO ANOTHER REALITY, where they are instantly transformed into COWS, when Superman DOUBLE-REVERSES their polarity by spinning around them with a cowboy hat, and collects all the cow energy from the UBER-MULTIVERSE, and channels it into them with help of reverse-cow molecule channeler he built from his socks and couple of trash cans.
They didn't need to say it's pc superman, it just is.
Do you know where the speed force comes from? In theory it is the sweat from superman, each droplet is the speed force of a Omniverse. PC Supes is just well pc supes, gg no re k thx bai flashes.