Credit: I got a lot of helpful information from the original post in this thread:
https://www.reddit.com/r/dynastywarriors/comments/xaroin/war...Pre-notes
1) The option to Craft weapons isn't unlocked until after you clear Ch. 5 (the end of the base game).
2) Crafting requires Materials, which aren't available in Story Mode (only via Infinite Mode, and also not unlocked until you clear Ch. 5).
3) Material drop rates are total crap in the 12 Zodiac Towers and Sky Father Tower (you basically have to wait until you finally unlock the Ophiucus Tower).
4) Get several characters to Max Promotion with a very powerful weapon or you are wasting your time and efforts.
Thus) Don't waste any time worrying about this until you are at the end game and ready to grind.
5) Finish all the other achievements, feel proud, close the game, grab and eat a snickers or whatever, and never ever ever come back to this stupid achievement.
So here's the guide:
Materials: Farm Ophiucus Tower with the team of 3 characters you want a weapon for (only the main three get rewarded). Clear the Tower run and profit an ... um, non-zero number of Materials. Then repeat over and over, about 3-5 times per team of three characters.
Gems: Constantly have full groups sent to "Training: Accumulation 2", farm the Ophiucus Tower (along with getting Materials, it's good for Gems), farm Story Mode levels on Pandemonium difficulty.
Crystals: Farm the Monster spawns in Chapter 7: "Gaia, Goddess of the Earth" on Pandemonium. (Other methods might be better, but I hear this is the best.)
Now, make a weapon for each of those 100 different characters and cry in joy that you are finally done with yet another unnecessarily-annoying "achievement".
---Conclusion of depression---
Or, be like I'm finally feeling after really starting on this last phase and realize that this is the one that is definitely just a trash heap waste of time that some developer should be fired for. It will take 3+ tower runs per set of 3 characters to make each of their weapons. A single tower run done ultra fast is about one hour (and that is speeding through each level and getting the time bonus so it is only 5 actual stages being done, so if you do all ten that's two hours minimum for each run).
That means that with the most optimal time possible, you are looking at 34 hours minimum to get this single post-game achievement for getting weapons you have literally no possible use for (as you have just beaten the absolute hardest challenge in the game). You probably cleared the rest of the game and most of the other achievements in that equivalent time. This achievement is one of those that is purely a waste of your time for absolutely no good reason and with no reward worth the effort. Seriously, just skip this insult to their fan base and get something worth your time like a nice ice cream cone in reality.