Heroes can level up through World map stages and Sanctuary Battles, but they can also quickly level up using Elixirs.
There are 2 types of Elixir of Growth. While regular Elixirs can be used on all Heroes, Peculiar Elixirs can only be used on a defined Hero Rank. Peculiar Elixirs have Rank limitations. (Ex. 3★ Elixirs can be used on only 3★ Heroes.) Peculiar Elixirs are rarer but allowed to level up faster, and using them cost much less than regular Elixirs.
Both the Elixirs can be obtained from world boss, Magic shop, Frodi's Magic Millstone and events, The can also be obtained from various missions, challenges and quests.
The Hero’s level can be raised this way up to Lv 30, and the higher Rank and level of the Hero is, the more cost the level up will be required. A maximum of 10 Elixirs can be selected simultaneously.