Check the number of Energy required for every stage, once you notice an increase, move to the previous stage and farm there for a while. Earn and use for Epic Hero Summons, or Special Summons (limited time). So it's best to only level up heroes that can be upgraded to at least Tier 3. Another Tier (green bar) next to the hero's Lvl will also lit up, showing how many times that hero can be upgraded further. The Hero's Lvl will reset to Lvl 1 but his or her stats will remain unchanged. Once you reach the required level, pay the fee and your hero will be upgraded. Upgrading heroes requires that you max out their levels first.
