Back to the raitaing with paper only Ionocidium.

Stats:
HP: Just enough to be over my 20k threshold. it's not bad, butnot really good. Just descent.
ATK: It's not a bad ATK stat but far from the best. Just in the middle.
DEF: Nice DEF stat, above the 3k so that's ok for me. But Iono is not espacially a defensive girl.
Stats are not bad but not especially good. It's plain average.

Skill
2.8x damage to all, 37% skill act.
A very classic all target skill. Good to handle... a lot a target. I have nothing more to say than I already said for other girls with that kind of skill.

Abilities
x1.2 skill rate
Nice to have like all the other girls with that.

+18% ATK for all
Good but below the average. Even some golds have a similar ATK buff (like her Christmas Ver.).

+10% skill damages for all
Here again, not that bad but still low. Usually it's +15% skill damages. Here it's more gold level (+8% for Iono Christmas or +10% for Creeping Jenny for example).

40% chance of re-act
Okay that's better. 40% is low when you know there is girls with 60% but it's better than the other abilities.

80% Evasion first two turns and 50% after
Very classic evade ability. But that's really interesting with her re-act ability. It gives her a longer life and more chance to use her re-act. Thoses two ability works well and benefits from the skill act and skill damage buff.
But that's not enough to make her really powerful.

Team Build
She is a bit to meh to belong in a solid high level team of any archetype out of an evasion team (and I'm not sure she can be called must have for an evasion team). But she is still useful and go in almost every lower level team for easier maps. For me that's her power. Not good to shines in very good teams but she brings a cool support with her Evade+Re-act for every teams a bit too weak.

Conclusion
Not a bad girl, not a good girl. She is average. She has an interesting Evade+Re-act combo but her other abilities are on the low side (even her re-act). She lacks of pure offensive power and good support for her friends. But her averagism make her really flexible to fit in almost every team when you still lack THE right girl.

Final verdict:
I guess she can be C Tier. For interesting stuff but she has too many low point in her abilities and stats and nothing special to really shines.