Undyne the Undying got stronger due to Determination, not LOVE, and Undyne not one-shotting Chara is probably game mechanics, PIS, or CIS.
Even though Flowey didn't destroy the same amount as Chara, it is incredibly illogical to assume that he couldn't when everything points to the fact he should very much be stronger. He has far higher love than Chara and 6 souls compared to Chara's 1 or 2, making him closer to Asriel than Chara ever will be. Also, there's the fact that when he gets the souls, the game crashes in the same manner as when Chara destroys the game, and the fact that the only save available is Flowey's World, and in Flowey's World there's nothing else but you, Flowey, your save file, and a distorted version of the opening, which Flowey likely only kept in order to mess with you. Other than that, there is literally nothing to speak of.
You keep saying that Sans is stronger Chara because they fought and won against them multiple times, which is still incorrect due to the fact Chara reset every time rather than simply regenerating a few feet away from him, but what does it even matter if Sans beat Chara if he doesn't remember it, hereby giving him no advantage because he doesn't actually know what happened? What does it matter if he beat Chara when the opponent he is currently fighting is literally infinitely faster than him when the most Chara can say is that they're Relativistic in speed if we include the Napstablook outlier? What does it matter that he beat Chara when the way Flowey fights is very much the opposite of Chara's, both in game and based on the visual representation of their fighting style?
To elaborate, Chara only swings their knife once every turn, then Sans gets his turn, which is why his Special Attack did anything at all, and even outside that every time they've killed a boss they've only used a single knife swing, minus Undyne, where the fight plays out very similarly to Sans' fight. Flowey attacks with an unpredictable display of attacks constantly in a reality where turns are nonexistent. Such attacks include a giant blue beam of death, nuclear warheads with his face on them, giant flies flying towards an opened venus flytrap, resetting your position mid fight to throw off your ability to dodge (The thing Sans needs to do in order to survive), and even the souls he uses to empower him.
I don't deny that Karmic Retribution would destroy Flowey. However, the assumption that Flowey would get hit at all is preposterous since he's Bloodlusted and is Immeasurable in speed and therefor would be able to effortlessly dodge Sans' attacks.
