I played the game on medium with a bunch of characters (unlocking characters) and here's what I've gathered about Seth. Probably not anything revolutionary...
With Seth, usually you get a free jump-in at the start of the match because he'll either throw a sonic boom or do a long standing hard punch. After that, be very careful with jumping in because he's good at dragon punching or even Ultra-ing you. Never jump in when he's knocked down. He'll just dragon punch you.
Also if he's knocked down, don't stand next to him as he'll just pile drive you. Don't try to do a meaty attack (an attack that hits him just as he's getting up) like a fireball or any normal, because if he has EX meter, he'll just do EX legs through it. I've had pretty good success just standing outside of pile drive range, blocking whatever attack he does (usually a normal or his Legs), and then retaliating while he's recovering.
If he knocks you down, it usually sucks because he teleports to different sides and then tries to dragon punch or pile drive you while you're confused and aren't sure which way to block (I get confused anyway). So try to avoid getting knocked down, and if he does knock you down, just try to get the hell out of there.
Chrono I'll definitely try to find you one of these days on PSN. What times/days are you usually on?
Sandwichcookie you're on AT too? I just recognized you from SRK, but your name looked familiar.