First and foremost, what you need to know is the basics of speed farming. There is a quick raid shortcut function by pressing the ` button(below Esc) on the keyboard. This works in battle only after beating a specific group of enemies for the first time. The game will recognized the specific groups of enemies you have defeated and hence quick raid is enabled for these groups. You can just press ` for an instant win even when you're being ambushed.
[This is L1 + R1 if you're using a Playstation controller, or LB + RB on the Xbox controller]

The best place to grind for money is in an area where it's only accessible if you're able to beat Bahamut and have Xiaohong the flying dragon. Defeating or losing to Bahamut will show the 1st ending credits, with the main difference being, defeating Bahamut will let you continue on your adventures while losing will mean NG+. So you have to beat Bahamut to be able to access to that area. To find Bahamut, go to Sakaki Central Mountain.

[Longitudinal Road West/East -> Guling Yamaguchi Lake -> Celia's home]
After entering Celia's house for the first time, exit to world map and talk to Celia. Then from Celia's house, exit to Guling Yamaguchi Lake and go south, then west across wooden bridge and up steps for a scene. Get Dragon Bell. After using it, you can fly to the center of the map to enter "The sacrifice of joy and sorrow".

Upon entering The sacrifice of joy and sorrow, the merchant you see sells every type of seed except magic moonlight grass. These seeds either cost 3000g or 30000g each, increasing your stats either by 3 or 30 each time. Hence the cost per stat up is 1 per 1000g. The enemies here drop 1000g each and sometimes drop a colorful diamond, recovery grass or soul flower which can be sold for 5000g, 1000g and 1000g respectively. You can further increased the rate at which you gain money by having the required skills and having enough starting SP or SP cost reduction accessories.

You will need howl, rain of arrows, fast walking, Treasure stance and festive moment outburst. Howl is to increase the encounter rate. Rain of arrows is to build up BP fast. Fast walking is to move fast. Treasure stance is to increase item drop rate. Festive moment is used to earn 5x exp and money while making all items drop 100% for that battle.

You can also consider pet mail order skill from monster class for the convenience of selling items. Monster class(Warcraft) can be found in Qinghe Town East inn and requires (Tamer lv30, Wizard lv30).

Howl - obtained from Thief class (found at G Material Sand inn, requires Hunter lv10)
Rain of arrows - obtained by using Triple Arrow + Triple Arrow + Curved Shot. Curved shot is acquired from Archer class found in Elf Village(Tesulin) item shop and requires (Hunter lv10). Triple Arrow is acquired from Magic Arrow class found at Ice Sheet after beating a boss in the cave. Ice Sheet(Tent) is found on the world map north west of Xue Xu Fortress.
Fast Walking - automatically obtained by leveling Survivalist.
Treasure stance - automatically obtained by leveling Survivalist. Found at Sakaki Port and requires (any basic class: Warrior lv10, Wizard lv10, Hunter lv10)
Festive moment outburst - obtained explosive skill from Survivalist.

[Passive SP Skills]
SP reserve 1 - automatically obtained by leveling Wizard (one of the 3 initial classes in Sakaki City)
SP reserve 2 - automatically obtained by leveling Magician (Tesulin house left of item shop, requires Wizard lv10)
SP reserve 3 - automatically obtained by leveling Magic Warrior (house in Green Forest Village, requires Berserker lv30, Magician lv30)
SP reserve 4 - automatically obtained by leveling Magic Arrow (Ice Sheet as mentioned above)
SP reserve 5 - automatically obtained by leveling Great Sorcerer (Examine book on table in Celia's home - select El, d, i, a)
Storage Stance(initial battle SP+50) - automatically obtained by leveling Great Sorcerer

The base SP cost of Rain of arrows is 30SP. To use it at least 2 times in battle without using any items, you will need a few or all of these passive SP skills(mentioned above) and accessories.

[Required Accessories]
Miracle Ring (all skills SP cost reduced by 1/4) - given by Celia by reaching 30 favorability
Lord of the Rings (all skills SP cost reduced by 1/3) - complete Mission 14 and open sarcophagus
Stone of Eldia (all skills SP cost reduced by 1/2) - given by Celia by reaching 70 favorability

Demonic Badge (initial battle SP+5) - (I forgot where this was obtained)
Demon Necklace (initial battle SP+10) - (I forgot where this was obtained)
magic ring (initial battle SP+15) - (I forgot where this was obtained)
Demon Crest (initial battle SP+20) - given by Celia by reaching 50 favorability
Magic Orb (initial battle SP+40) - hidden room in island west of Snow Firefly Village(requires flying)

Any 2 combination of these accessories is good enough as long as you have the required SP to cast Rain of arrows 2 times consecutively in battle.

[Make sure you have defeat every group of enemies at least once. To also make this effective, your main hero must not be DEX focused build as you don't want rain of arrows to wipe out enemies in 1 turn.]

Once you have all the required skills, head to 2F of The sacrifice of joy and sorrow and start farming for money. Sometimes there will be a group of 8 enemies appearing, this is the time you use the festive moment outburst and followed by quick raid to end the battle. Every quick raid you use builds up 10 BP. To build up BP fast, use rain of arrows on the group of 8 enemies. You can gain 48 BP this way. Use it twice and your outburst will be ready(since you get 10 BP from the previous battle). This makes farming for money and colorful diamonds(worth 5k each) relatively fast. And you can build up thousands of stat gains in an hour.

Cheers and have fun grinding stats.