LogFAQs > #949973988

As far as early high tier weapons, the most notable is Sheik's level 4+ Harp (4+ is the best tier) is available super early on the Koholint map, I got it at like level 45.
Sheik's already one of the most overpowered characters, and with such a strong weapon, she can be your 2nd character on really tough stages and make it a breeze.

[LFAQs-redacted-quote]

You're really not meant to 100% games like this.
It's an ensemble cast, Koei knows people will have their favorite characters and others they won't touch. It gives plenty of content to use the people you like. Though requiring characters on a lot of adventure mode battles goes against this (but a lot of times you can take 2-3 characters now and just not use the required one).
Also don't recommend playing all of it in one go. I usually play one game at a time, but a game like this is honestly better to do in spurts. Put dozens of hours in, go to another game, come back, and so on.
Trying to 100% it is fucking crazy. Just play it and work towards unlocking stuff you want until you get bored, then drop it and maybe come back later or don't. It'll still easily be 100+ hours of fun.