Let's get into some arguments for Itsuwa.
First: I wonder why it is assumed that Ouma is a lot more skilled than Itsuwa. Itsuwa is one of the head combatants of the Amakusa, meaning she has trained for a long time to become an elite magician and fighter. Their goal was to be able to fight alongside
Kanzaki Kaori, which means they had a rather high goal. And they managed it. In the fight against Acqua the Amakusa as a group were able to fight at the side of Kanzaki without being just baggage or something. In other words they are so skilled that they aren't easily annihilated against a monster like
Acqua of The Back (first key), despite being so much weaker.
In addition to that spear users have a inherent advantage in battle against sword users, due to weapon compatibility. Under equal conditions (skill, stats etc.) spears usually beat swords. (
Cool video on that)
That aside Itsuwa has another advantage: She can freely disengage to prep or surprise. Reason for that is that like any other magician in the verse Itsuwa can create People-Clearing Fields. That is she can choose an area and make it so that the subconsciouses of all people around are manipulated so that they will not enter that area. With that she can at basically any point retreat, heal up and prep. And she can make use of that prep to for example cast her Scattered Gras spell, which vastly improves her defensive abilities. This also gets her out of his oxygen manip, by just leaving its AoE.
With that said I want to direct attention to the reasoning of the "much higher" durability stat with Damage Transfer Charm and Scattered Gras: She could block
Acqua of The Back's physical strikes. We are talking about 1st key Acqua here at that. If she gets off her prep she can tank what Ouma throws at her for a very long time until her magical clothes are destroyed.
Without the prep the Damage Transfer Charm bit is still in place and should still give her considerable tanking ability.
And her procedurally getting nude will distract Ouma.
Next let's get to his air defense shield. I'm not sure if it block Itsuwa's White Explosion magic. First, this magic creates two explosions. One at the tip of Itsuwa's spear and the other right at the enemy Itsuwa is facing. I don't know how tight his barrier is, but if it isn't directly on his skin the second explosion might happen within the barrier, making it useless.
The second way the explosion may bypass the shield is due to its inherent properties. It is a spell based on the sun, that produces heat and light. Air conducts heat and doesn't affect light at all, meaning it can go past the barrier more or less unimpeded.
So, regarding Ouma's stat amp: Considering that that 100% goes into 8-A I assume this is restricted. Though I think it needs more to completly blitz Itsuwa either way.
Vacuum blades can be countered with stell wires. It doesn't really circumvent the damage transfer charm either way.
Let's talk about the Mandala as well. Illusions are not just good at hiding, but also in fooling opponents about what is there. If it looks to him like Itsuwa is defending and tanking vacuum blades, there is no reason to start using AoE stuff or searching for her true form. Worst case she could even make an illusion that Ryuuzume is broken, knocking him out. Though that's of course just one of many ways it could go.
When not using illusions, she could insert a thought into his head to make him think that one of his techniques was analyzed and that he hence won't use it. That way she can get rid of whatever proofs most troublesome, like the air barrier, invisibility or oxygen thing.
Summa summarum Itsuwa could to some extent hold herself against heavier hitters than Ouma and has a toolbox that can let her win this.