I’ve had great success power leveling my heroes. I’d like to share some of my methods that have helped me and I hope can help you. This takes commitment and time. Have a game plan and know exactly what npcs fields you can hit the the most each hour. You will be looking for fields that have either 5s or 10s within 2 miles. Any further is wasting your time. Remember time is key. Your game is not your base but your hero!
For low level heroes trying to reach a point where they can hit 10s what i do is i search for a base that has several lvl 5 npcs around me that are very close. I will bounce this hero to as many npcs within the same hour. Base defense regenerate hourly. So repeat this over and over.
Bouncing 5s with warriors-
Another thing people like to do is add warriors to their attack to increase the experience that is given to their hero. Some view this as a waste of resources, however you cant deny the fact that your hero does level faster. You be the judge.
Ballista / archer combos-
This is typically done when your hero is too weak to farm 10s solo. I warn you to be extremely cautious about this as it can really cripple you early. Using 9k or more ballistas and a good archer rainbow you can afford to use heroes as low as 120s. This is risky so use a war horn to make sure you win.
This works well getting mid level heroes to a point where they can go solo. If you have a hero capable of hitting 10s or a teammate you can work together to level. You send your best hero to hit a lvl 10 followed by a 2nd hero you’re trying to level. The first hero will break defenses down and 2nd hero will hit after with minimal losses. This is very effective.
You should have a hero that can hit 10s solo for this. As in bouncing you find a base with several 10s in close proximity. You this time do the same thing but you send with NO transports. The key is speed. Hit as many 10s in one hour as possible and then repeat. If you’re concerned about resources and other heroes you could have a hero shadow you with transports. If you are concerned about archer losses/resources losses trust me when I say that though this will be difficult it will get much much easier as your build times increase later and well well worth it! Some players will even move their heroes to multiple locations within the same hour to get the most for their experience.
This is what myself and many have done and it all works. Good luck and happy leveling. I would suggest you follow my path for leveling your heroes.