For the skeleton, you have 5s after the decay to kill them, and it take multiple fire ball. Other wise, you can hit them insted of casting fire ball. For your problem of no being albe to cast, it’s because you have a mana bar. Over 100, a dacay use 25, and a fire ball 10. Your mana restaure by itself, but realy slowly. The game is not finish, but normaly, you would find some mana regeneration items.