I think Ciella's Challenge is way too difficult, I can understand gathering balls alone with a bad weapon card, but the amount of enemies and gigabytes it throws at you are way too much for it to be plausible in my mind, I can barely get two out of the three balls needed to even beat it the first time (I haven't even been able to beat it and I sure as hell am not wasting levels on good cards bashing my head against it until I do), and any amount of enemies I do defeat are always immediately met with many more dropping at once, there is no time to actively open the firewall while holding off a force that relentless, even with gigabyte resistant upgrade cards, Meira's Challenge is much more fair in comparison
If you wanna a tip, try a bit magnet card so you can get a lot of bits quickly while you kill enemies. With 150ish bits collected, you can get the ball quite fast, fast enough so luring enemies to the other side of the map gives you enough time.
But yeah, I would agree, Ciella's Challenge feels way more difficult and unfair compared to Meira's Challenge. I think the biggest problem is the never ending pression. If it take a little longer to enemies respawn, we could get a little of pression off, and have more time to get the ball. For sure it would feel better.