Yeah, you should try and keep Seth at about half a screens distance. From here, he'll either use stretchy arms or throw a Sonic Boom at you. Whichever he does, just avoid it with j.hk, then either sweep, or go into a combo of your choice.
When he's knocked down, create a half-screen gap again.
If Seth starts his teleport Shenanigans, just crouch and mash jab.
This works for all characters, apart from Gen and Dhalsim.
With Gen, jump in with medium-kick and mash s.hp. If he teleports, use Oga.
As for Dhalsim, I don't know how the hell I managed to beat Seth with him.