Look Slade has been beating Wally west sine he was kid Flash and even when he grew up. Th identitiy Crisis fight was merely the latest in a string of defeats suffered at Slade's hand. The tactic he used to tag wally is the same one Double Down uses.
Anywho this fight comes down to a matter of durability. How durabe is Ben Grimm.
First of all it has never been stated that Slade's sword is promethium. I know that it is really strong and I think it might be promethieum. The staff I know is promethium because it stood up to Superman's heat vision. Slade is a 2-5 ton or so character so unless his sword cuts Ben like butter its not gonna happen. He can porably win by using hsi staff ver and over again to wear Ben down but its an uphill battle for Slade. Of course the thread originiator said he gets those meta busting grenades so if he does then hsi chances go up dramatically. Of course that seems alittle unfair.