I have been playing for a while. I'm level 58 but my hero's are just at level 30. I have seen members that are level 40 something that have hero's that are 40+.
1st keep in mind that you need to spend souls on Transgress every 10 lvls to increase their max lvl and let your hero continue advancing.
Now as for getting exp...
In Solo Campaign find the highest hero only mission you can beat and do it as often as possible. While Solos are not as likely to give exp as a resource raid, they do occasionally give small amounts of crystals and souls (25c and 70s usually).
Then do resource raids as much as you can and target the weakest players you can find. Don't worry about resources, just make sure you can 3 star the base. Even better if you can use heros to beat them without using any units so you dont have to wait for them to train. Use the crystals you earned on Solo to heal heros and attack again.