I just checked the chapter.
Fern did not see the demon and immediately fly kilometers away like you're suggesting she will do here.
Of course she didn't, she couldn't untill she got a chance to, a chance she'd have right away here because her foe is slow af compared to her and lacks the means to actually prevent her from doing what she wants.
She did that because of the fog which made her unable to sense the demon's attacks and because the fog also acted to alert the demon of her attacks allowing them to block it.
And Yuta has sensing and deflection does he not? If you're gonna argue the long range mage who always stays just out of her foe's range if possible won't actually do so because "she only went like 20km because of this", that actually still applies here.
So what's her go to against characters that can sense and block her attacks?
You guessed it, you even elaborated.
I also just skimmed the solitar fight. Fern shot her from maximum range as a sneak attack, to make sure she didnt sense it coming after she thought fern was dead.
Please for the love of god don't say skimmed. I can't take you seriously, you legit just outed yourself in ignorance anyway by saying Sol thought she was dead.
You say sneak attack while ignoring why it worked to begin with, that being "attack from a distance your foe cant sense or react to you". And why did she do it? So Sol couldn't react to it, because she was out of Sol's range.
And no, Sol knew Fern was alive, in fact she had already beat the shit out of both Fern and Stark together previously, and she knew that were out there still because El Dorado was undone. She just lied to Frieren about them being dead to piss her off. Fern simply got the attack off because she was out of Sol's detection range, so she couldn't react in time.
When she first encountered he while she flew it was not nearly at such heights that was first presented. It may not be in character for yuta to just look at his opponent and say die first thing but is it actually in character for fern to look at her opponent and fly kilometers away?
If possible yeah. She tries to stay out of her foe's range, that isn't always possible if she's blindsided or fighting characters with even greater range than her own, but if given the chance, that's her go to. Saves on mana from making shields, puts her at safety, she's weak against CQC fighters such as assassins as they usually hold stat advantage in physicals, etc.
From the three presented examples she had actual reasons to do so for two of them(idk which chapter she fights this assassin), reasons she wouldnt have with yuta.
She'd have even more with Yuta actually, she's been trained to stay out of range of CQC fighters as much as possible. Straight up never let them get close to her type deal, otherwise she'd die, is what she's been taught.
Meanwhile yuta has a reason to use cursed speech due to the dozens of zoltraaks that are faster than him per second.
And Fern has reason to do the thing she does constantly, I gave you examples of kms, but her creating distance and flying away? Happens almost every fight she's ever been in, the demon and sol fight just happen to be the biggest examples that eclipse Yuta's range by literal 100x tho, but you could just read the manga or watch the anime to see how she usually opts to go ranged.
But nah, your argument is legit, "Yuta uses cursed speech to-", when the battlefield is gonna be constantly carpet bombed so have fun hearing what's being said.
You assume he can do this despite a hefty speed gap giving her ample time to react to and even retaliate accordingly to anything he attempts to do, and all while he still has to deal with countless attacks that might even one shot him if she decides to damage boost before he can go max output, which you'd know if you just skimmed Sol's fight, that shit is hefty boost.
You mentioned Ladder but that only works on CE, which isn't equalized to mana, and her attacks have piercing to that stuff anyway by 1 layer, which is, in fact, listed on the magic page.
And the rest doesn't even make sense because she's just so much faster than him that any other argument doesn't really work.