As I said, I feel that your party deals with Ultimecia before Time Compression is complete, otherwise, you wouldn't be fighting other monsters on your way to kick her ass. By the time that your party battles her final form, Ultimecia is absorbing time and all that exists is you and her, something that is stated if you scan her. I think you can resist TC for a limited time but once the spell goes into full swing it would be impossible to surmount. I think TC only ended once Ulty got killed.

Omega is also sealed away, which to me tells me that he is powerful but beatable. A full TC Ultimecia could probably destroy Omega since TC itself would not destroy him to begin with, rather just freeze him in time.