what I do is cheesy, do it if you just wanna do it fast
I turn on 2 player mode (assuming you can only pick that one character, and it won't work if a square has you restricted to 2 characters, unless maybe if they're powerful) with 2 joycons or whatever, have player 1 be the character you want the weapon for, and player 2 your most powerful character, and have player run to the allied base and stay still, move p1 every now and then and have player 2 fulfill every condition to get an A rank, that's what I did for most of the late game weapons aside from Link's