Woohoo

Well, Discord might not be able to harm Aku, but it's possible he could find a way to seal him away forever, or at least a very long time. Aku might not be hurt or anything but if he can't break out of the prison he's still losing. Aku's pure evil and rather powerful himself, but he's not shown reality warping abilities to the same extent Discord has.

Aku is capable of making time portals though, and I suppose portals in general. I dunno. c: I feel like the best Discord could really do is run away. Maybe.

True, but for all we know Discord can trap him in an Anti-Portal prison. Which is stretching it but Discord's powers lack much definition. He can rip open portals himself, and the comics have him also capable of Time Travel.