In the newly implemented Explore system, the difficulty now ranges from as low as Level 11 or Level 18 (about as hard as the old Explore battles on Normal) to Level 200 to 250 at Difficulty 12. This is a significant challenge to surmount, as much as a level 250 Delve.
However, all these battles of higher difficulties still only reward one Class XP to the equipped hero. This makes progressing slow and frustrating.
On the other hand, there is legitimate concern in the idea of making the lower difficulty battles provide full XP.
My solution is to make a threshold; currently players get a Seal bonus for doing battles above Difficulty II; this indicates that the devs believe this is the threshold at which battles get difficult. Providing bonus Class XP at this threshold, or even giving 3 or more Class XP per battle at higher levels, would be greatly appreciated.
One way it could be done is that Class XP is based on a formula. Class XP Gained = (Highest Level of Enemy Defeated) / 10. This means that a normal level 20 battle gives 2 Class XP, low-difficulty Explores give 1, and Delves (which are significantly more difficult than normal battles) would give bonus XP.