About 3 lvl 45 green give same exp.as 1 charm, best for you to farm adventure for fodder whether it's for leveling characters or equipment.

Charms are very, very, hard to farm. You will most often have to rely on using blue gear or fodder gear (the stuff that drops in adventure) instead. I personally find charms drop from side story a little more, but that's not a proven fact. Otherwise, you'll have to rely on Huche and the one goblin in 3-4 to buy their stock daily. You can set your High Command to get you ancient coins and/or Conquest Points. In the shop ancient coins can buy neck and ring charms, while CP can buy the others. The less likely option is to do a ton of Arena to collect the CP for the same arena shop. 

Go to Lab 1-1 every day and visit Huche's shop at the back of map. It resets every day. Buy every charm he has.

Buy neck and ring charms using Ancient Coins farmed from Lab and Raid.

Level the other four slots using fodder from story mode. You should be farming story quite often for catalysts and levelling dogs anyway, the fodder gear is everywhere and cheap to feed-so it's my go-to source for upgrading gear.

I only really need charms for the neck and ring, and you have Raid and deployment missions to farm the coins needed.