The level "Into The Underdark" has unlimited experience and gold. When you get to the part of the stage with the ice barrier and wizards, switch to Zhai and use the shadows on the right side of the bridge to sneak attack enemies that appear from the right tunnel. You can do this as long as desired. Zhai steals gold from enemies when invisible. Get as many kills as possible. When satisfied, continue on to fight Cireka. Only kill the wizards that fire missiles at you. The large slow moving bombs go directly for the character you are currently playing. An unlimited number of enemies will flow from the portal. Kill them and get your characters' hero meters up. When full, use it repeatedly. You must do this at least twenty times for decent experience. The more times you do your team attack, the higher the experience multiplier at the end of the level.