There's various spots to farm XP in the game. Here's two of them, accompanied by a video I made for each one.

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Here, at Barta Crags in front of the Greatwall and the Greatwall Encampment you can find a Wight at night, a creature which eternally spawns undead enemies with high XP reward.

Go there, consume XP boost items, and start farming them without killing the Wight. If you kill it, it will not respawn until NG+, so try to go with no range-magic pawns, only melee so they can not reach it.

Generally they will reach the Wight anyways, so if your pawns are getting close to killing the Wight, try to escape and wait a day or sleep for it to respawn with full health.

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After the quest A Warm Welcome, every Gran Soren guard at the Castle Grounds and Noble Quarter will be hostile towards you. They will keep spawning, there's no limit on that.

As shown in my video, you can find one or more spots where no guard could reach you. Get there and let your pawns do all the hard work (Guards are not hostile towards them) or kill them by yourself. You can even go AFK here.

Each pawn you have with you will take part of your experience.

• For an enemy that gives 100xp:
  
  3 Pawns = 100xp
  2 Pawns = 100 x 1.2 = 120xp
  1 Pawns = 100 x 1.2 = 120 x 1.2 = 144xp
  0 Pawns = 100 x 1.2 = 120 x 1.2 = 144 x 1.2 = 172.8 xp (shows as 173xp).

So if you want to farm XP, you might want to be alone or with the fewer pawns possible.

However if any of your pawns is 25 levels below your character, it will not cost you any XP. So if your Arisen's level is high, you may want to get a few pawns with great utility/damage skills and 25 or more levels below you.

Weal is a status effect displayed as small four-leaf clover next to their name under the item menu. It doubles Experience, Discipline Points and Rift Crystals earned by pawns.

You can get the Weal effect from using one of these items:

Gain Weal for five minutes.

You can buy it from Barroch at Bitterblack Isle.

It can be found inside a Treasure Chest within the Everfall at Ceremonial Cage, Chamber of Absence, Chamber of Fate and Chamber of Hope.

It can be found inside a Treasure Chest at Bitterblack isle in The Fallen City and The Black Abbey.

Gain Weal (Double XP) for 5 minutes.

Can be stolen from Gran Soren Soldiers, Salvation Members, Wandering Pawns and certain Bandits using Master Thief.

Aelinore rewards you with 3 Veteran's Periapts for escorting her to the northern Healing Spring in the Notice Board Quest Beyond This Stony Cage, initiated from the Union Inn in Gran Soren.

May be found inside a chest towards the northern edge of the Witchwood, at the area before Selene's Cottage is reached.

Can be found as loot inside sarcophagi located around the northern Healing Spring in the Northface Forest.

May be found in a chest in The Ancient Quarry.

You can buy it from Fournival at Gran Soren during Post-Game/Stage 7 if you have high affinity with him.

Gain Weal (Double XP) and Prosperity for 5 minutes.

You can buy it from Fournival at Gran Soren after the Off With Its Head quest.

You can buy it from Madeleine and Aestella after killing Grigori, at Post-Game/Stage 7.

It may be found around Soulflayer Canyon, and inside chests in the DLC chapters Garden of Ignominy and Gutter of Misery, from Bitterblack Isle.

When carried in the inventory, the flower grants Weal; after one ingame day the flower will transform into Dead Flower becoming useless.

You can get it from Quina during the quest Talent in Bloom.

A perfect copy can be forged at The Black Cat for 180000 Gold.

Gain Weal for five minutes and the Curse status effect, which reduces Strength, Defence and Stamina.

You can find it in a chest at The Blighted Manse, near to the rear entrance of the sewers.

You may find it in chests at Bitterblack Isle, in Fortress of Remembrance, Rotwood Depository or The Forgotten Hall.

Can be stolen from Harpies, Skeleton Mages, or Skeleton Sorcerers, by using Master Thief.

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Also Hardmode doubles all experience for you and your pawns.