User blog comment:FairyRaven/Fairy Tail Chapter 363: The Stories That Demons Read, Review and Commentary/@comment-1693281-20131207185050/@comment-7418318-20131208133806

If he's fighting back and notices before she can seal his magic, sure. But with her ability to completely nullify an opponent's ability to use magic, she actually does have a better chance to defeat him than most other mages do, if she can catch him by surprise.

But that's assuming Jellal will fight back in the first place, rather than just letting Mili kill him so he can atone for his crimes. Remember that told Erza he'd be fine with it if she'd kill him for what he did (which caused the slap and tumble that led to the almost-kiss).