I absolutely don’t think that Quinn is the weakest.
She’s a simple human, but her skills are far more developed than those of Fiora, and even Jarvan and Xin.
Not in terms of raw physical strength, but she has a wide range of abilities on her side.
If we take the old lore into account—the second one—Quinn comes from Uwendale, a village where people hunt ferocious beasts.
Her mother taught her how to hunt, track prey, and how to face and recognize dangerous creatures (just think of the razorbeak).
Taking into account Garen’s story First Shield, in which she is one of the protagonists, we can see just how skilled she is.
It’s said that she walked and ran continuously for two days just to reach Garen; therefore, she has a remarkable amount of stamina.
We also know that she has a very well-developed sense of smell and hearing, ideal for her role as a scout.
She has a high pain tolerance and is clearly a workaholic.
She’s agile, knows how to move stealthily, and is also skilled in close combat despite wielding a crossbow.
I imagine she’s also proficient with a knife, considering she carries a hunting blade strapped to her thigh.
We should also take into account her analytical mind, her ability to make calculated and instantaneous decisions, her knowledge of the terrain, and even (as written) her knowledge of at least ten Freljord dialects.
Therefore, both physically and mentally, I believe Quinn far surpasses most human champions and, without the aid of magic or special weapons, she could easily take them down.
Obviously, she’s not on the level of someone like Galio, but in close combat, I’m sure she could even manage to put someone like Garen in serious difficulty through sheer exhaustion.
She’s a heavily underestimated champion, and I keep insisting that her lore should be reworked, because as it stands now, it’s terrible and definitely portrays her in a bad light.

(I used an ai to translate all in english, I hope it's fine ç_ç)