XP Island
XP Island is an island where you can collect free XP potions for 0 stamina cost and is unlocked at level 20 and is similar to the Gold Island in that you can only access it once a day and choosing a stage will lock the rest of the stages regardless if you complete it or not.
XP island is inhabited mostly by nature element units so a good amount of fire heroes should do the trick. On each stage, there are different elemental enemies scattered throughout them but can be dispatched easily leaving the Nature heroes you have prepared for.
Quick Tip: The sparkling objects can sometimes give hero tokens for the Phenol Thoxian jelly hero.
The final boss of each area are variations of a caster type enemy. He will use poison related attacks and debuffs like silence to limit your use of skills. Defeat him and you may claim your XP potion reward that is based on the type of chest and a Phenol Thoxian hero token.