Many people recommend setting the game to 1 round & 30 second clock to increase your odds, but often Seth doesn't need 30 seconds to KO your ass.
A cheap-but-easy way to pummel the bastard is to use Russia's Red Cyclone, Zangief. Not only has his strength & hitbox been improved for this game, his moves have very high priority. This means that you can spam some of his stuff and still get your hits in.
To easily conquer Seth (and almost anyone else except Cammy) in Arcade mode on any difficulty, simply abuse Zangief's Spinning Lariat. This move is done by pressing all 3 Punch buttons, which by default (on the Controller - Type B setting) is mapped to your Left Trigger. The Right Trigger presses all 3 Kick buttons, which is a quicker, shorter version of his Lariat.
At the beginning of the round, get to 'within sweep range' of Seth and use the full Lariat. Seth will likely block the first hit or two, or begin a Focus Attack, but the full Lariat hits enough times to break through this. Once you've knocked him down, use either Lariat as he begins his get up animation. If Seth manages to get away from you, wait for him to do a jump in or his teleport-to-throw routine and use Quick Lariats when he is close. If he starts to do his double kick maneuver, begin a Focus Attack (Medium Punch + Medium Kick) and, depending on his timing/animation frames, either let the Focus Attack complete or cancel it and spam another Lariat.
Rinse and repeat until the bastage is down!