Farming

Well, farming is essentially doing a quest over and over to get a certain amount of the same item. For example, you need 10 Eternal Gears to max ascend Okita so you can farm the Chicago Free Quest on America to get them. It may be boring but during idle time between events and chapters, it is the best you can do.

And I advise you, if you want to farm, go for the materials that can drop (Bronze and silver ones) or QP/EXP cards. Farming for gold materials like Hearts is a waste of AP.

A farming team is a team specialized in finishing Free Quests on the minimum turns possible. So you need servants with AoE NP to clear the waves. You may need a servant with a good single target NP with enemy like Spriggan, Gazer... You need also a CE that gives a starting NP for fast clearing.

One of the best servants for farming is Lancelot : you take him with a kaleidoscope CE, you activate his 2nd skill, you blew the first or the second wave and you have +30 stars for the next wave. Saber Alter and her Rider counterpart are also very good. Any servants with a good attack stats and an AoE NP will do for farming.

Farming, in gachas/other games, refers to the act of doing a repeated activity as quickly as possible to get certain items. In FGO, this usually means either ember farming or ascension/skill material farming. So the goal is to create a team that can do these activities as fast as possible.

The top servants for this as those who have strong AOE NPs that can clear these waves without problem, and those with NP charge skills are even more valuable. Some good servants for this are ones like Drake, Sparticus, Waver (to charge others skills), etc. In the future, there will be some more additions to f2p farmers like Medusa getting her 3rd skill, and the same for Arash.

Key Craft Essences for this are the NP charge ones with the top tier one being Kaleidoscope. With 3 Kaleidoscopes and a support waver, you can 3 turn a lot of farming nodes without issue. Other good ones include Imaginary Element and Dragon's Meridian among other NP charge CEs.

An example of my ember farming team is:
Waver (Bond EXP CE)
Darius III (Kscope)
Eric Bloodaxe (Kscope)
Kiyohime (Kscope)
Support unit (Bond EXP CE)
Random unit to gain free bond exp (Friend point CE)

Now obviously not everyone will be as lucky as I was, so I'll list some other units who can fill the roles:

Waver substitute: Helena is the only unit I'm aware of who can charge other units NP charge at the moment. You can use the Mage's Association mystic code however to charge someone's NP by 20%

Craft Essenses: Kscope > The Imaginary Element > Halloween Princess > Dragon's Meridian

Units with NP charge skills:
5*: Drake, Tesla, Ryougi Shiki, Karna, Arjuna, Morded, (Gilgamesh in the future)
4*: Nursery Rhyme, Astolfo, Edison
3*: Babbage (Medusa in the future)
2*:
1*: Spaticus (Arash in the future)