Gold Island
Gold Island is a special island that rewards players with a decent amount of gold upon completion. It is unlocked at level 25 but the challenge is the recommended level is 30. You don't need stamina to enter a stage but you can only play a stage once, regardless if you beat it or not so choose wisely.
Gold Island is inhabited by dwarves and they are comprised of the Fire and Dark Elements. Most of them are armored as well. Them being dwarves, they have the Perfect Swing trait that renders their attacks impossible to miss. Meaning bringing heroes that rely on dodging or skills that decrease enemy accuracy like Daze won't be effective on them due to this passive skill.
It is recommended to bring a Water and / or Light team to effectively counter their elemental weaknesses. You could also decide to bring heroes with the hunter traits like Dwarf Hunter or Warrior Hunter even if they don't match the opponents weakness. For example, Jabber Clenchjaw is a Nature hero. He will be at a disadvantage versus the Fire element. But having him attack the Dark element dwarves will give him a suitable damage output in the party.
The final boss is a beefed up version of the dwarves you have faced. Always check how many stars the opponents have to see how difficult the challenge will be. The later stages will offer the same enemies but have a greater number of stars.