Using Warlord IV on every battle, 100% soul armor, and VIP 5+ with capped Valk casts in every fight netted me roughly 8575 souls for clearing out a kingdom. Clearing out every challenge in the game using this method will get you 205,800 souls. Of course, about 117,600 of these souls would be from just using Valk in the fights themselves. The actual “bonus” souls for first time challenge clears across all stars amounts to 150 * 7 * 24 * (your soul modifier), which with these numbers would be around 88200. While all that might sound nice, keep in mind that it usually takes around 25-35k souls total to reach a 3* in a given kingdom, and over 1.3 million to level all current troops to max, a number that grows every week. Clearing new challenges, even with full soul bonuses and added VIP bonuses, barely puts a dent.
Arena’s soul reward is not high enough to be viable for “farming” souls, especially once you have hit two ascensions on Valk and have level 10 in every magic kingdom, where Valk farming becomes much faster.
The official word was they were working on something for this issue, though. The best idea pitched from the playerbase was raising soul rewards based on the current rarity of the killed troop, bringing the baseline reward for full mythics up to about 50-60% of the cap without using Valk. Hopefully they will do this and not just add generators, which would still be very restrictive in the teams you’d be willing to play with if you wanted to progress.