Simple guide on how does AFK-Farming work in Granblue Fantasy: Relink, including Endless Ragnarok.
・Setting up your character.

In the same Gameplay Settings page you’ll find the ‘Skybound Arts Activation ‘ option, set it to ‘Immediately‘, this way your NPCs will do their Skybound Arts whenever they can (Instead of waiting for your character to do so).

You can (and probably prefer) to change these settings back to ‘Off‘ and ‘Full Burst Only‘ respectively when you’re manually playing once again.
Keep in mind that the main way to avoid damage that your character will use during Full Assist Mode will be a Perfect Dodge, and not every character is entirely balanced around this mode. Some of them will try to strike a charging attack but will get stuck in a loop of ‘attack, dodge, attack, dodge‘, especially in harder difficulties where bosses will attack more constantly.
The best Party Leader you can use while AFK-Farmingwill mostly be range-focused, such as Eugen or Tweyen; but you can go with some solid melee characters just like Charlotta or Zeta (You might want to take away all of her skills, this way she’ll do her air-loop more consistently and you can also give her the Less Is More sigil), or supports like Cagliostro.
Quests difficulties from Maniac onwards will be easier as long as your character has a bit of Tank on its build, which can be achieved by not only specific Sigils like Aegis/Greater Aegis, Stout Heart, Steel Nerves, Health (You can stack them up to 99999 HP) and Autorevive, but also by having their Terminus Weapon unlocked and equipped, as they come with Regen Lvl. 15. You can also give them a Sigil with both the Potion Holder and Auto Potion traits so they can heal themselves (They won’t use mega potions, but they will still heal a lot by using regular potions if you take those two Sigils with you).
The Crabvestment Returns Sigil , which grants you with 10% DMG Reduction and a 20% +Max HP, will be extremely useful for AFK-Farming in harder difficulties, and to obtain it you’ll have to pick all the Crabs during the main story quests and then speak with Lil Pince at Folca.

・Basics of AFK-Farming.
You’ll have to play or wait through an entire quest once until you beat it and get to see the post-game summary, where you’ll be able to toggle Repeat Quest.

While it’s activated, you’ll be able to go afk and the quest will repeat for a total of 10 times. Every 10 runs you’ll have to come back and do it all once again.

It’s. That. Simple.
・Preferable Quests for AFK-Farming.

Basically any quest can be AFK-farmed with powerful-enough characters, even Proud ones, so the basic idea would be to prepare your characters and then search into the rewards that any quest provides, so you can personally pinpoint which quest you’d like to AFK-farm.
So I’ll name only a few quests that are popular between the AFK-farming catalogue, but you can do whichever you’d like.
- “Slimepede” (Maniac – Survival).

- “Assault Formation” (Extreme – Survival).

- “Gulp… So These Are the Rumored Monsters” (Extreme – Survival).

- “Shedding Light on the Pillar” (Extreme – Defend).

- “Death, Taxes and Goblins” (Maniac – Boss).

This quest will reward you with Broken Savage Swords.
- “Golemology 103: Forest Golems” (Maniac – Boss).

This quest will reward you with Crowning Parapets.
You can luckily obtain Silver Centrums by finishing both of these quests normally, but even if you don’t, both previously mentioned rewards (Broken Savage Swords and Crowning Parapets) can be exchanged for Silver Centrums by doing Treasure Trade with Siero’s Knickknack.
- “Delts of Our Lives” (Hard – Boss).

- “Golemology 103: Forest Golems” (Maniac – Boss).
This one again. It will also reward you with Curios.

・Extra: ‘Futureproof Trading’ by the Gifted Merchant.
This dude at Folca is the Gifted Merchant, he’ll provide you with a Repeatable Quest called ‘Futureproof Trading‘.

He’ll ask for 5 Fire Shards and 5 Earth Shards.
- You can obtain Fire Shards by beating Vulcan.
- You can obtain Earth Shards by beating Gallanza.
So you’ll have a lot of these items to give him. And to repeat this quest, all you have to do is just enter Lyria’s Journal, which works as a loading screen, meaning that once you exit the journal, he’ll provide you with the same quest once again.

EDIT: Okay this one does not work the same since Endless Ragnarok released.