I read some info in other threads, but I didn't find anything really useful.
Does anyone have an effective method to increase the weapon mastery level? Because at S level it goes up extremely slow, I just soloed Ragena Ruins with Asmodeus where there are tons of monsters, but it only went up by let's say 1/5 of the entire bar... it's gonna take days just to get her to use her ultimate weapon, I can't imagine how much time I'm gonna need for every single character... am I doing something wrong? Did someone manage to get all/most of characters to M level? How did you do it? Any tips is appreciated, thanks!

My characters all have M. As this is the second run, it doesn't include Roka, Evelyn or Lyphia.

I think most got there automatically as I tried to keep characters somewhat at same levels on the first round. Only been a few times I've had to level up that attribute, and those have been from partially filled S to M.
Even for +NG characters, most got there automatically too when I leveled them to 99. Even characters like Hannah (well, you get her at lvl 1, and if you level to 99 afterwards, it's pretty bound to happen...Though I do wonder what place I used as she has ~900 kills in her stats, might perhaps been a lower level deadbeat floor).

Most characters seem to get to M at around their ~500 kills, this is a guesstimate and may vary depending on the character but overall seems to hold true. Might vary according to playing style.

200 hours according to save window, this round. As for first round, latest remaining save from it is from chapter 7 and shows 130 hours.

Kill counts for characters at this point (beginning of chapter 9, Yuela route) stretch from 475 to 999. (At the end of first round I'm pretty sure most had 999...)

Difficulty is easy. Though I'll probably do the 4th round at hard. Going to collect everything Emelita route first on easy.

Everything from first round that could be upgraded or found was already done before this round (well, at least everything on Serawi route, clothes, weapons, enemy stats/drops etc).

Best way I found when you need to kill a lot of monsters is just to use the Special Arena. The Rank M arena monsters are still generally one attack kill even without stat caps. Even easier at lower ranks.
Of course, playing anything but the hardest difficulty is too easy in any game for me, so I speak from hard mode experience.

I generally just take off animations and turn on end turn automatically (the only place I ever have it on) and whack the hell out of those melkaveils.

Weapon Mastery needs like 21,000 to level from A to S

From S to M you need 30,000.

You get roughly 100 points per attack round, from what I gather. More for crits than regular attacks.

You dont get any points if your unarmed... using bare fists or bites. Also unsure if killing a mob gets you more points or not. Defending or Healing gets you nothing... you must attack even if it's "Green".

You can always cheat and use the trainer... it got boring really fast trying to level it up so I used trainer.

Thanks for your inputs guys. I'll see if I can figure out something.

Here are some hint :
1. Go to Demonic Banquet
2. Stay on the middle base
3. Use your weakest melee weapon
4. Don't move

This is applicable for everyone except Black Eushully