Meria's challenge is too much.
I feel the same way.
I'm not usually afraid of a challenge, but losing card levels to the time restraint of the challenge keeps me from wanting to attempt it as often (at least, with any good cards). Defeating 100 bytes isn't bad by itself, but the time control with Meira's challenge is brutal-
Recently, I got a 91/100 score once after playing that challenge & I've had a hard time getting close to that since. As of this post, the only way to increase the time in this challenge is by defeating the fancy colored bytes (impersonator, basher, keyboard warrior, etc.), which gives you +2 seconds to the clock. This doesn't help much since it often takes longer than 2 seconds to kill most of those fancy bytes, even if you focus entirely on them with your weak weapon. Thankfully, you still have bombs to help with big groups, so at least you're not entirely hopeless.
Even still, the RNG between where those fancy bytes are at any given time & whether or not they die from the bombs exploding lends itself to time waste & a likely diminished high-value card which will put you in a slightly worse standing to win for the next run.
I'm not saying that Meira's challenge is entirely impossible, but I am saying that it's very likely to eat away at your cards &/or your time investment in this game. I love this game a lot, but I am worried that challenges like this could drive people away from getting the full achievement experience.