Get Hero Shards from the dungeons in the single player campaign. They are used to recruit heroes. Complete the dungeons to get as many heroes as possible quickly for them to build up experience with each battle. Even losses in battle will earn experience. Grind previously completed dungeons for increased rewards and Hero Shards. The Arena rewards you with Honor Badges, which can be used to get new heroes or improve the ones you already have through sacrifice. Sacrifice unneeded heroes to improve the stats of other heroes. The better the hero, the more experience you will earn. Only keep the best possible heroes active.