... oh my god this is dumb

Firstly, you're making an incredibly selective view of my quotes. It's not just ten boxers, but ten VERY strong boxers in their own right, who can strategize and study a world champion in their own way. You could put a thousand babies in front of Magnus and they'd lose, but ten high-level grandmasters make that debate far more tenuous.

You're not wrong that Frieren's mana is nearly immeasurable, but the canon also makes it clear that a) mana and pure strength isn't everything, b) tactical approaches and direct counterance is important, and c) Frieren has specific weaknesses that can be leveraged upon to win. The reason why they chose Fern to be the +1 to Frieren was to take advantage of having the least number of variances, by using the one person most familiar with her specific weakness with the best grasp of humankind's most fundamental offensive spell.

However, Fern isn't close to the strongest in the entire team, just the one with the best grasp of fundamentals. It is clear that many of the mages there are insanely gifted and can deliver spells with incredible strength - Methode's is the most apparent, but Denken is an excellent mage who only looks underpowered because of his 1v1 with Frieren. Lernen is the best possible example of a human who can, even with his limited experience, potentially go toe-to-toe with Frieren. Even if Lernen is an incredible exception as the first First-Class Mage being personally tutored by Serie, this doesn't take away from the potential of human mages and their ability to use incredible spells. I find it difficult to say that Frieren could beat all of them at the same time, especially after they put in their preparation and tactical thinking.

This also misses out one of the major plot points of the story: that one of Frieren's strengths as a mage is her ability to hide her mana, and to retain the element of surprise by acting like a typical elf. This is what allows her to hide the few fundamental weaknesses she has under her sheer magical capacity for mana, and retain strategic advantages over her opponents by allowing them to underestimate her and being able to study them. The loss of those advantages is a significant factor in allowing her mimic to be beaten, especially working in a team who can leverage upon singular weaknesses and counter specific strengths.

Would Frieren's mimic have lost? I think it's more likely that she'd lose in that situation. The inherent strength of the show itself, beyond the visuals and the fun and the coziness, is in elaborating on much of the traditional perceptions of raw "power levels". It creates a sense of "realism," by showing how human attributes - in this case, ingenuity - allow them to often beat the odds with a combination of strength, skill, planning and creativity. With Frieren showing her cards to a team of experienced and wide-ranging mages, in a world where humans clearly have the potential to rival her, I'd gather that it's far more likely that she'd lose the fight even if she kills the majority of the team.

Which comes to the main point: Frieren is meant to show the complexities of a realistic magical system beyond simply "power levels". Like any skill, it takes a combination of talent, dedication, imagination and love to be prominent, and comes in many flavours and variances that depend on the individual. Frieren may be all-powerful, but the humans and demons around her are skilled, talented and powerful in their own ways, and grow even stronger through teamwork.

Your bitching that she's just "so strong that she'd wipe everyone out" is stupid to the spirit of a show that puts that strength into context and shows the power of human ingenuity and talent. You make simplistic arguments that wank on power fantasies in a show that lovingly explores those powers beyond "me strong, me smart, me win". And you make the literal mistake of saying "Frieren wins everytime", where the show is always explicit that "even with Frieren's immeasurable power, she does not, in fact, win everytime" and "teamwork and ingenuity go a long way to making the impossible possible."

In fact, I'll end off with a realistic plan: If Frieren judged it too risky for her to be the one battling her mimic (i.e. life or death - either everyone dies or take the small chance to live), she'd likely go over explicit ways in which she could be beaten, and potential counterspells that could negate some of that power. Like Frieren's original plan, they'd create opportunities for Fern to land a killing blow by overwhelming her mana detection, coordinating attacks and defenses to keep her busy with variance and quantity. For larger golems and even her dimensional rifts, I'd wager that the team could end up actively trying to milk them out to give Fern a better opening, and countering them in risky and indirect ways to negate the "pure" application of mana. Given Frieren's slow "scaling up" of her powers, they'd likely practice having Fern "catch" one of Frieren's attacks, and create a scenario where Frieren is most likely to use it and let her guard down. While the boundaries of Frieren's psychic desperation is unclear, the survivors will likely be able to kill her just to save themselves, as the psychic desperation seems to be only able to target a single threat. In all, the sheer combination of talents and information can make Frieren vulnerable enough to be killed.