Okay, allow me to throw in some opinions.
Theoretically, there are two kinds of gameplay that should work for Slayer in this matchup:
1. Minimal movement,
2. Maximum movement.
With minimal movement, you basically try to play Yomi games all the time. This could work, because Millia's movement is too godly for Slayer BUT her attacks aren't. Beat her attacks, land your big combos. Just remember that if you wanna beat her attacks, you gotta understand the hitboxes of the attacks, because Millia is one of those who has many deceptive-looking attacks (look "big" but actually beatable, you know what I mean?).
With maximum movement, you will have to maximize your skill on dashing, especially ground dashing. Dandy Step FRC's are useful too. 90% of the time you will move around, and you will always try to move NO MATTER WHAT. You will only rarely try to beat her attacks (not to worry, since Slayer doesn't need to attack a lot to hurt her, obviously). Slayer who fights like this is hard for Millia to apply guessing games on. And since guessing games are her biggest assets....
I think there is practically no useful gameplay for Slayer between those two. If you move around a lot but also try to play Yomi to beat her attacks, you're dead. If you Yomi to beat her attacks but you don't focus on it because you like to move around too, you're dead.