Ibuki is one of the characters that I think can really give Chun a hard time if you’re not too familiar with her. She can go high/low and take your health little by little, and has a good jump in game for more opportunities. Ibuki players also see you way more than you see her, so it may get a little rough at times.
On the positive side though, you only need a few more opportunities than Ibuki does. I would say to start off, your defense have to be on point. I never get hit by her f+mk for any reason at all, UOH catches me sometimes, but I’m usually focused enough to block that on reaction if in a tournament offline.
If you want to guess in between strings, or know when they will attack, go high. Her main source of confirmable damage is s.mk xx forward slash kicks against you. She cannot short or EX slash kick you. Most Ibuki players know this but if you see it more than once, you can always low forward and super the EX slash kick or short one if you’re fast enough, but during the short one you only have 1 slash kick to do it.
Going low vs Ibuki is not the best think like against most characters. She has a lot of “off-the-ground” normals, so if you go low too often and are only looking to hit that way, you’ll be having trouble getting around some of her normals. If you’re close enough, you can jab her out of her f+forward, f+roundhouse, UOH etc. If you want further range, s.strong is a good move to zone her and keep her grounded. If you see a f+roundhouse and ducked it, you can also tag her with your low roundhouse. If you walk back from her f+forward, you can trip guard her and confirm into super, but the timing is very strict.
In the air, you either want to chase her with a jump double fierce, or just block if you can’t do that at the moment. Spread eagle works nice too and is safer than jump double fierce. You can also dash under before she tries any mixup from the air.
Keep in mind most Ibuki players set up their jump with f+forward, jump over crossup mk, and they may mix up there or jump again, it depends.
When you shut down their “off-the-ground” game, they will start to go low to hit your back fierces, strongs, jabs, whatever. This is when you can play some low forwards as well, and where you have the big advantage.
If Ibuki does st.forward -> command dash, jump back or forward over here, wherever is away from the corner. It beats both grab and instant SA1, but keep in mind jumping towards her can be beaten by an early standing roundhouse, but usually jumping away is the safest option. Just that if you do it too often, take the grab instead of risking getting caught.