There are two weapons in EarthBound that are incredibly difficult to find. Not 'oh, you have to beat this powerful boss' hard or 'gee, this puzzle sure was tricky' hard. More like 'I have to spend five hours fighting the same enemy over and over until the random number generator causes one to finally drop' kind of hard. And only one of those is even remotely worth it.
Both the Sword of Kings and the Gutsy Bat have a one-in-128 chance of being dropped when you defeat a two specific enemies. The Sword of Kings is worth the grind (as much as any sort of repetitive task for a fleeting, digital reward can be worth it), as it's the only weapon Poo can equip that doesn't cause massive stat drops, and you've still got a third of the game to go by the time you nab it. No, it's Ness' ultimate weapon, the Gutsy Bat, that is totally not worth the effort, as you have to fight the incredibly difficult Bionic Kraken a gazillion times before it drops - and by the time it does, your attack power will likely be maxed out anyway. Besides, you're already on the last dungeon, so it's not like it'll get much use. You could spend that time trying to develop your own latent psychic abilities and it'd feel like less of a waste.