She has a 50% chance to dodge in the impossible difficulty, though there are also some special rules to prevent being very lucky or very unlucky:
-She can't dodge twice in a row, so if she already dodged once, the next attack will surely hit her;
-She can't be hit three times in a row, if you hit her twice in a row, she'll surely dodge the third attack.
For the normal mode these special rules are different.
Edit: Also she won't dodge after the final attack.
Viewing post in Undyne the Undying fight remake comments
Yeah, I checked the code for those special rules, and there's actually a ">" instead of a ">=" in the part where it checks for the hits in a row, so right now you can hit her three times in a row in impossible mode and four in normal mode, thanks for making me notice this bug.
The part with the number of dodges seems right though, she can dodge only once in a row in impossible and twice in normal.