That is still not a power mod. Again, when Shantae's light magic changed to dark magic, she did not gain any new abilities. All it did was make her evil. That's it. This isn't the first time it's done something like as when dark magic is being infused with the Tinkerbats, this also makes them go against Risky Boots and attack her.
It isn't exactly that difficult to sneak up to someone when they can't see you. I'd argue it would be easier for Shantae since Natsu did this by being too fast for her eyes and attacking her from the back. Shantae however can just simply turn invisible, and attack her in any direction without getting caught. Shantae's invisibility is via an item, we don't see her use them in cutscenes because whenever a fight happens, you play.
Let me adress the win-cons for Shantae.
Healing - If Shantae takes too much damage then she simply fully restores her health while Ultear can't. Shantae can keep doing this over and over until she eventually wins.
Starting Distance - With Warp Dance, Shantae is going to close the gap between Ultear, allowing her to start the fight and attacking immediately.
AP - Shantae already has the 2.5x ap advantage, with Twin Mints doubling her power, making it a 5x ap advantage. Then Super Monster Milk will give her the 5.6x ap advantage, and yes I tested this out. Her base damage was 8, using Super Monster Milk made her damage to 18. So if Shantae uses these, Ultear is going to die
very quickly if Shantae uses these and starts to attack her.
Teleportation - Shantae can (and has) used her teleportation in combat. If a large hoard of orbs come at her than Shantae can teleport away from them. While these aren't exactly the same thing, Shantae has been able to dodge
projectiles in small spaces, as well as
gaint canonballs and
balls that chase her.
Transformations - Shantae's crab form can also just block the orbs via her own
shell. Her spider venom can also possibly affect her as usually a spider's venom is basically their spit. And since the blood somehow couldn't allow Ultear to reverse it, this might be the same case for her venom. Other forms such as monkey can increase her speed and make her smaller, with mouse form making her even smaller.
Explosion: Even if Ultear can negate the explosion, I still doubt she can react to the first one. She will not be aware of what Shantae's abilities are, and since her dances can do several things, I doubt she'd be able to react to the first explosion. Since her ability is not though based. This gives Shantae at least a single easy and strong hit.
Items - Vanishing cream allows Shantae to be completely invisible, so she can simply walk to Ultear, and she won't know specifically where Shantae is. Even if she throws her orbs in every direction, they would only phase through Shantae as she can completely
phase through objects. Bikini armor also halves the damage Shantae takes, so this allows her to tank more hits.