When Rinslet fought Velsaria with the other girls, it was her first time fighting such an opponent; she didn't have experience like that before. Yes, they were 4 fighting Velsaria to compensate for the power gap, but it's not any less an important experience for Rinslet. Later, in Volume 13, Rinslet fights off ice dragons, that could fly too. If anything, being a flier makes it easier for Rinslet to aim and shoot.
If sealed-Yoshino is 384 tons, and unsealed-Yoshino is much stronger, let's assume for the sake of the argument that she's right on the upper echelon if 8-A: 1 kiloton. As we are taking Rinslet as her best, let's use what her profile is based of off: Volume 13 (I still have to read the rest). Between volumes 1 and 13, Rinslet has gotten stronger (no shit), but since we don't have a number, let's assume she's at x2-x2.5 stronger than baseline 8-A, which is what is assumed for Volume 1, for safety purposes. Yoshino would be about x5 stronger, and it will certainly hurt Rinslet.
With Field of Cold and/or longer casting time/wave of arrows, she can decrease the gap a bit. Yoshino would still win in AP, but Rinslet's attack speed is much better's than the other equally-strong characters (despite equalization in speed, if attack speed scales higher, the scaling remains). Yoshino's power null works against physical attacks, while Rinslet's attacks are magical, including her arrows, so they won't be blocked. And like I said before, she has access to healing and is smarter.
I know Yoshino is a good fighter, especially when unsealed, (I've seen the anime) and I could see her beating Rinslet, but I'm bringing up these things to point out Rinslet isn't helpless or without options here. I'm not sure myself if her advantages will be enough to beat Yoshino, but at the very least, she has Yoshino beat in intelligence, range, regen/healing, and power control (it's stated Yoshino can't fully control her Angel form)