Playing against computer players should not be used as a test to see if you are ready to play online. Human players are able to adapt and change their strategy in response to what you do. In turn you may be able to catch on to their patterns or tendencies and punish them for doing so. The best way to see if you are ready for online is to play online. If you find yourself losing a lot, look at your replays and see what you are doing wrong. If it is something like executing a combo, then you can go to training mode and practice until you have it down.
Computer players can only play in one way, the way they are programmed. Because they don’t play like the way humans play, they don’t make for good opponents. Have you noticed that the computer sometimes get you with an ultra, special move or a poke just right after you press a button to do something. That is because the computer is reading your inputs and instantly executing a counter. It is most apparent in arcade mode Seth. When he is low on health or just for the hell of it, he will beat out whatever you are trying to do. However he is also made to give you openings by doing unsafe moves. For example, arcade Seth would often randomly throw out the kick that does multiple hits when it connects, but it is very punishable if you block it. He would also randomly attempt a jump-ins, use his stretching arms at point blank or use an ultra. All of them on block are very punishable.
So if you really do want to beat arcade Seth you have to heavily punish him on every opportunity he gives you.