General

You can farm those trait potions for 400 reboot coins each during this event.
Those give a lot of exp to one trait.

Buy gollux belts, and fuse.

the fastest way to farm charm is to von leon and fuse gloves.

PB with a billion droprate

Zakum helmets, Horntail pendants etc.

Trait potion from event hall. farm it with different characters and then transfer it to your main. ezpz

People here are just amateurs when it comes to free charm exp.
Other than the typical bosses, try hunting down the Kachuu Musha and Emperor toad at kaede castle.
Specially the Kachuu Musha a.k.a. Red samurai thing, It drops a lot of draconic equipment worth 60 willpower and charm
imperial equipment which gives 90 charm and grand amethyst equipment which can give 50 empathy, charm, or dilligence depending on which piece of the set you get.
I was getting over 1500 charm exp on a daily basis hunting these on x2 drop with greed pendant, no joke.

@hopelessworld: My charm level is 65 and I already have a pocket slot. I'm asking this for the facial expressions. Thanks for the tip though! (Definitely going to do this on my other characters now)

I do this:
zak*2
Ht*2
And normal Von Leon once
I exchange the VL coins for gloves and then fuse them. (Collecting the Purification totems is boring af)
Took me a week or so To reach lv 30 charm doing this...

Royal hair a lot

Do Monster Park (you can buy 5 tickets a day) the mustaches give charm and other traits

Red Leaf high, whenever it gets here, also sells a pocket slot in the shop. It unlocks your pocket slot without having to go through lvl30 charm and rose clipping.

@icefrappe: Once I max my diligence in another like 30 days or so I'll make the switch.

@helpingly: That stinks I usually make all my secondary characters that I think I may play long term, an alchemist to start. Practically speeds up the process of getting to level 30 charm for the pocket slot 1.5x faster. That aside, I think Ranmaru's equips also give charm as well, so just buy out your class ones in FM whenever you see them if they're cheap

@icefrappe: Totally forgot about the alchemy fusing technique. Too bad I'm already a meister craftsman.

As most people have stated already, czhelms and chtp have been nerfed to 1 per kill. That said, if you don't care about actual exp too much and just charm, then go do normal zak for up to 2 zhelms per kill and normal ht for up to 2 pendants per kill. Pendants will have to be dropped in horntail's map after equipping before you can pick up another one. On the other hand, you can hold more than 1 normal zhelm in your inventory (unlike czhelms) and I suggest you hold on to them. By learning Alchemy, fuse normal zhelms together for another 50 charm.

As stated above, gollux, perm nx, C/RA, and empress can all yield you charm as well but I think out of that list the most cost efficient method is doing daily gollux runs and RA. There are chances for gollux pendant/belts to drop when you kill gollux's head, and there are chances for the RA bosses to drop their respective hats. Not to mention, cracked and solid gollux pendants, belts and the RA hats are probably the cheapest items with charm to buy from the FM AND you can hold more than one in your inventory which means you can fuse them after they have been equipped to get even more charm.

I used to get 3 czhelms per czak but now it's only one, same with cHTP. however, you can still get more than 1 zhelms, and each is 50 charm so one run usually gets at least 2 for me so already better than czak, but the exp is a lot worse

-2 czhelms a day for 140 exp (used to be 4 but it got nerfed post Reboot)
-2 or 3 gollux belts and pendants a day for about 200+ exp
-2 cht pendants a day for I dont remember how much