In theory this isn't too difficult, or that time consuming. I got it in a day, and that's with using a variety of characters for most of it. If I realised it earlier, I could have saved myself an hour or two...
To make Weapon Master a cakewalk, use Nightmare. Keep using his 66B attack (dash/double-tap forward and Y). For some reason, the AI just sucks against this move, and it will win you a good 90% of your fights. It's also a fantastic way of getting the '10 Perfect Victories' achievement. It's not a perfect method though, there will be times the AI will kick you down, but this method is the quickest/easiest way I found. In terms of weapons, switch to Flamberge (damages through guard), but switch to Soul Edge (Growth) when you can. The health increase is worth it. And finally, forget Soul Edge (Complete) and take up Soul Calibur when you have that available.
There are some fights where it doesn't work so well though. In the rounds where you need to hit the opponent a certain amount of times, Nightmare's a bit too slow, so switch to someone faster (I went for Sophitia). The rounds where you need a ring-out also caused me some trouble, but using Nightmare's 4B (back and Y) seemed to get the job done, I also switched to Xianghua and spammed her 4K (back and B).
The stage that gave me the most trouble was Chapter 4 Stage 3, Merope Monastery. This is the one you need to play when you reach Level 72 to unlock Subchapter 2 (and by extension, Lizardman). You face Mitsurugi, Voldo, Taki, Nightmare and Cervantes. The problem? You don't have a lot of time, and Taki is pure evil. The rest go down to your 66B shenanigans like anyone else, but Taki doesn't. She will guard and not let up. But, there is a move Nightmare has which for some reason will nearly take her out in one hit. Use AAB (X, X, Y), but leave a pause before you press Y. Try it in Practice and you'll get the hang of it. You'll see a blue flame around Nightmare's weapon if you're doing it right. You hit Taki with that and you'll take off nearly all her health with it.
If you've done everything but still haven't hit 72, there is a way to quickly grind up. Chapter 6 Stage 2 was my choice for grinding. You have 5 opponents, and you can choose to battle the next to double your gold, or leave after one opponent and keep the gold you earnt. Do this, and leave after the first fight. They're quick, easy, and you get a high amount of XP for such a short fight.
Overall this achievement is fairly easy, and not that time consuming if you use Nightmare right off the bat. Took me about 5-6 hours of solid playtime to get it. In doing so, I also earnt these achievements: