Here is a few Quick Tips on what adventure to farm for fast souls / XP 

If You want to Grow your Partners then you need Souls to enhance it and you probably do not want to waste gems to buy souls because you can easily get it from playing adventure follow my tip to get best results.

The fastest way to Collect Souls

If you’re looking to make “Souls” AKA the purple stuff used to enhance partners or war patterns, and you want to maximize this amount for your energy farm the forest final level on purgatory difficulty. The Badger armor sells for 1800-3000 Souls + you get 1650 XP and 1804 Souls for winning the battle.

Another good option for this is the final level of Ice Lake. You will get slightly fewer Souls for the armor around 1500-2500, but 1800 XP and 2004 Souls for winning the battle.

A 3rd great option is to farm the 6th level of Ice Lake or Forest as this only costs 5 energy per attack, allowing you to get 3 extra attacks in for 90 energy (Assuming lvl 25). Which leads to a potential to sell 3 more armors, and get 3 more bonus XP/Souls per 90 energy used, gives nearly the same as the boss level!

It is best to use One 6* or one of your best partners to ensure you will win. If you have a new 6* that needs leveling this is great also! Then use 1-3 partners you wish to farm levels on. Adjust as needed if failure occurs. Overall this will help you not waste potential XP. (Pro Tip: If you have a 6 Star BNK He can solo any level (if you built him correctly), on Purgatory. You can farm XP for 3 partners regardless of star or type with him.)

Update 2019: If you want to farm for Soul only then Last adventure stage (Magic Feild) Normal level (10-6) will give you around ~2121 Souls and Owl armor cost will be around 700-900 Souls. This stage will cost you only 3 energy. So you will get more rounds and more souls as compared with any other level. But I would not recommend this stage if you want to farm your partner as it doesn’t give much XP also at this stage hard to farm more number for 1-3 Star partner.

Special Thanks to Sean Barry 

P.S – Don’t Spend Souls until you are so desperate to do so. I would Suggest to wait for Soul event so that you can get Extra Benefit.