The sole example I can think of where a weapon isn’t otherwise available except from a strong enemy would be the lynel weapons, which let’s face it, are end game anyway. Plus with a little practice you can cheese the lynels and not even run down your weapon durability.

But more to the point, pretty much every other weapon in the game has set spawn points each blood moon. You just need to mark the most interesting ones when you need a powerful weapon. The botw one that comes to mind is the flame greatsword that spawns right inside the waterfall entrance to the castle.

In totk, pristine weapons spawn in abundance in the chasms.

So really it’s more about knowing where to gather them outside of fights.