I was playing chun li vs seth earlier and seth’s overly generous jumping arc is a real pain when you are lacking a strong and dependable anti-air, even worse when you also contend with lag. I could see the cross up every time and blocked against it quite often but I kept getting picked off by low light attacks and whenever I blocked both I was in a non-stop cross-up cycle that I couldn’t get out of, I couldn’t seem to hit focus dash quickly enough either.
Dash forward under the cross-up then punish with a safe combo or if you got a forward dash that goes far just sweep.
Sometimes you can also low profile cross-ups, but that’s rather risky.
If it helps any, it was brought up recently in the Chun forums. Some of these vids may help. This post onwards: "Becoming the strongest woman in the world!" Chun-li Match-up Thread
Learn to block better/adapt to the lag if it’s online basically. Then learn Chun’s anti airs and how to antiair/combat Seth better.
The anti-crossup demo (third vid here) might also help. Chun-li Q&A: Ask simple questions here!