You're missing the point. Setting aside the fact that I think the idea of beating a big bad by using shapeshifting magic into something innocuous is actually a cool concept that's a lot better than just murdering the **** out of an eldritch abomination, the issue isn't with non-sequiturs. You (and others) keep giving that example, but that's not at issue here.
Frodo was across the Ford.
But the pursuers were close behind. At the top of the bank the horse halted and turned about neighing fiercely. There were Nine Riders at the water's edge below, and Frodo's spirit quailed before the threat of their uplifted faces. He knew of nothing that would prevent them from crossing as easily as he had done; and he felt that it was useless to try to escape over the long uncertain path from the Ford to the edge of Rivendell, if once the Riders crossed. In any case he felt that he was commanded urgently to halt. Hatred again stirred in him, but he had no longer the strength to refuse.
Suddenly the foremost Rider spurred his horse forward. It checked at the water and reared up. With a great effort Frodo sat upright and brandished his sword.
'Go back!' he cried. 'Go back to the Land of Mordor, and follow me no more!' His voice sounded thin and shrill in his own ears. The Riders halted, but Frodo had not the power of Bombadil. His enemies laughed at him with a harsh and chilling laughter. 'Come back! Come back!' they called. 'To Mordor we will take you!'
'Go back!' he whispered.
'The Ring! The Ring!' they cried with deadly voices; and immediately their leader urged his horse forward into the water, followed closely by two others.
'By Elbereth and Lúthien the Fair,' said Frodo with a last effort, lifting up his sword, 'you shall have neither the Ring nor me!'
Then the leader, who was now half across the Ford, stood up menacing in his stirrups, and raised up his hand. Frodo was stricken dumb.... The foremost of the black horses had almost set foot upon the shore
Now tell me, why couldn't Frodo drive back the Ringwraiths? Why could he not scare them off with the name of Elbereth? Why not turn them into trees? WHy did he need to be rescued?
To use the kryptonite example, the issue isn't with it existing or being hokey as a concept. It's saying that fiction should have some in-depth treatise on the chemical features of kryptonite, and then arguing that it's not really consistent with the nonsense chemistry rules they invented for kryptonyte to behave in X or Y way.
le rules or formula is a pro, not a con.
It doesn't need to be overly detailed, it just needs to be consistent with the setting. If kryptonite is said to have a particular qualities, then they shouldn't later be told they have a completely different ones.
For example:green kryptonite is supposed to weaken Superman. But if being exposed to green kryptonite seemed to do nothing, I'd want an explanation.
And good fantasy sometimes doesn't have rules at all - the Song of Ice and Fire series gives us nothing on how the rules of magic work. And we have insane stuff like having sex with a witch let's her create murderous shadow baby regicide machines. In the right hands - a good writer - the fact that magic has no predictable rules is a pro, not a con
No, magic does have rules. The pro to it is that its rules are not bound by our rules. But they have to have a form of internal logic behind it, or the story loses all cohesion because literally anything can happen at any moment.