Make dark elixir storage immune to lightning spell
Currently, there is no defense against someone dropping several lightning spells on top of your dark elixir (DE) storage to steal your DE. It means you can steal DE with zero skill and high return on investment. Instead, I would simply make the dark elixir storage immune to the lightning spell so that it requires skill to steal someone's DE.
Alternatively, the dark elixir storage could be made immune to lightning only when upgraded to the maximum.
Goodheretic