This is a reeeeeeally close matchup. Both of their ice resistance is amazing, with both capable of no-selling AZ attacks. Mitsuru can lower the resistance to ice, while Froslass has an advantage in versatility via having more options other than ice compared to her.
Spirit Drain and Draining Kiss counter each other, though Spirit Drain. I can't tell if it's an ice skill or a spiritual one, and depending on which it either becomes half or twice as effective.
Mitsuru can summon her Persona(s), giving her a numbers advantage. While Snow Cloak will make Froslass a pain in the butt to hit, only made worse by Dodge and Double Team.
In terms of status effects, Froslass has that one in the bag. Mitsuru has moves that inflict Panic and Charm, but they're not common to inflict. While same could be said for the flinching moves, you can't for Will-O-Wisp or Confuse Ray, which are guaranteed to inflict if they hit, and are very useful as well, as confused, Mitsuru is attacking herself instead of executing an attack, giving Froslass more than just a free turn, and burns do continuous damage along with noticeably lowering attack. Furthermore, Frostbite's an ability, not attack, so it's not getting nulled. And Froslass' freezing has shown to last years. And with access to Hex, she's doing double damage due to those statuses
In terms of statistics amplification, Mitsuru has the edge, hilariously for the same reasons Froslass took the above. Froslass has less options in it, and while her one is good, the boost doesn't happen often. Meanwhile, Ice Amp is useful, along with Mind Charge. Debugging on the other hand, is back to Froslass, as Mitsuru only has the reduction of ice resistance. Froslass has the defense lowering Leer and the Sp. Def lowering Fake Tears (which is more useful as it drops the stat twice as much as Leer, and Froslass is more adept in her special stat), Crunch and Shadow Ball have a chance of lowering Def and Sp. Def respectively, and the aforementioned burns. And if Dodge activates, there's the defense lowering there as well.
What really matters will be their miscellaneous trump cards not mentioned in my analysis above. The elephant in the room is healing, which is...pretty good. Megidolaon is a strong Almighty attack. Nuff said. Analyze is downright scary, being able to size up Froslass, including the weaknesses, and with her tactical advantage, won't be put to waste. The biggest threat however, is Vorpal Blade. Not only is it very strong on its own, but it's also super-effective. And she'll know because of Analyze.
For Froslass, there's the basic stat advantage, along with having far more range. There's the passive power nullification with Cursed Body (requires her to get hit by what's being nullified first), and active with Disable. Spikes restrains the earthbound Mitsuru as she's fighting the airborne Froslass. The biggest threat is Destiny Bond though, as it's a desperation move that's basically "If I can't take you down, then I'm taking you down with me."
Imo, after this analysis, I believe Froslass will win in the end. High diff. Let me start off by saying absorption won't likelycome into play here. Anyway...Both will start off with their ice attacks, and Mitsuru would have the edge first. She's taking no damage while Froslass is taking chip damage. And Mitsuru might realize that ice is ineffective first as well. But once Froslass realizes this, which wouldn't be long as she's used to fighting Ice- types, then the balance shifts. Will-O-Wisp is going to suck for her, as she's vulnerable to fire. Along with Confuse Ray, which lowers the use of her Analyze skill. While Mitsuru is capable of lowering Froslass' ice resistance, Froslass has more than enough attacks that aren't ice that don't require that setup, and those moves play off each other quite well, such as a status into Hex, or Shadow Ball and Ominous Wind spam to spin that wheel of chance. Mitsuru's stat amping is pretty good, but Froslass's debuffing is as potent, and has, while unreliable, a chance to gain a much more all-around boost in Ominous Wind, which damages with AoE as well. What Mitsuru's best chances here rely on are her Persona and Vorpal Blade. However, there are factors to take into account there as well. For one, the burn. The debuff there is everpresent. Second, there's the power advantage, meaning Froslass can likely shrug off one or two of those. And it could activate Frostbite, which is basically game over. Third, it could activate Cursed Body, nullifying it. Or Disable once Froslass sees it and realizes its threat. Fourth, Spikes setting lowers the movement. Fifth, because of Froslass's annoying evasion, due to Snow Cloak, Double Team spam, being able to outrange, and the chance of Dodge, and even flight or the confusion damaging chance, Mitsuru might not even hit Froslass at all. And if all else fails, Destiny Bond.
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