>>149056320Flash is basically just a "power jobber" at this point because they constantly have to show the audience he's not invincible and is unquestionably the best means of letting the writer wank a certain character off when they down him.
Deathstroke is the opposite, power-wise you'd think he'd be lower but he's THE "badass" villain of the DC universe and if he shows up, there's basically an obligation to make him do something incredibly badass and establish him as a big threat to our heroes. This has been consistent since the 80s when he waltzed in on Batman and beat the everliving hell out of him during the earliest years of Bat-wank. A moment which everyone really found cool at the time and which they keep trying to replicate.
As a result, if Flash and Deathstroke are in the same comic, these two principles attract to each other and thus, the universe itself dictates that Slade will always embarrass the Flash