I really like the ascension of the challenges and how the last 2 difficulties are usually very challenging/tough. However, playing on Warlord 1 in Arena, I can get just over 3000 souls/7000 gold for winning 8 fights. No real use for me to spend time gaining souls in the challenges, even with a strong Soul/farm team.
Suggestion? Make the Challenges relevant again, perhaps through a weekly event.