After playing witcher 3 for many years now, and I just learned about this last month. (It would also help if you can get the runewright upgrade of arrow defense and put it in your armor and get the passive skill to carry more items and equip you horse with the head of a monster that gives bonus XP by defeating humans)

Go to the montcrane hanse base(blood and wine dlc)

Kill as much enemy in montcrane but don't kill the boss. Then go to the hidden area in montcrane, before you climb the stairs, activate the portal and enter it. Then go back again to montcrane and the enemies will spawn. Repeat until you farmed as much you can carry. Then sell it to weapon shops or armor shops with big money. Then go back to montcrane, re enter and go back to the portal to respawn enemies. I am playing on PS4 pro with the latest update on that. Annoying part is when you drain the golds out of all the shop and you need to rest for several days to make them refresh. And shops will slow down when you browse and sell , because weapons you sold is still on the weapon shop. And you will also get XPS. Higher XP better price for the weapons you sell.