To level up our heroes we need Exp items (Wisdom Tomes).
I have looked in to all the ways we can get them, and here are the best ways:
1) Use Stamina
100 Stamina = 45k to 72k Exp (depends on which stage)
After running out of stamina potions, it costs 100 gems per 100 for 2 purchases, then cost 200 gems per 100 for 3 times. I recommend spending your gems here if that is your focus.
I spend 800 gems here daily.
2) Falcon III
The falcon gives a random item, either food, wood, gold, 20 enhancing stones, 25k Exp tomes, 200 Stamina, or 200 Mobility. So spending 150 gems to refresh it, will give you on average 43k Exp if you just count the 200 Stamina and 25k Exp item alone.
I spend 300 gems here daily.
3) lvl6 Tribes
Average of 14k Exp with Doug
Trade with resources, or buy for 30-60% off the market price of 10k EXP for 120 gems. I will always refresh once for 100 gems. You will need to reach castle 18, to get the better items. I mainly buy all the tomes that are 50-60% off here.
5) lvl25 Monster
Average of 3.6k Exp with Doug
There are other ways to get more EXP that you should take advantage of, such as VIP Store, Throne Donations, Alliance Store. Also, try to get as many summon tickets as you can as they have about a 20% chance of giving EXP items.