The more sand balls hit the mage's body, the more of its energies were being used up. As long as that is happening, the mage will also continue to get weaker and weaker
However, the mage naturally won't let that happen easily. Its body suddenly turned into a dark fog that filled the entire locked space
Cairo felt a dissolving power from the black fog and his Mana Cloak starts to get thinner as if the mana in them were melting away. This mage is really a tough nut to crack and is not an easy enemy
Despite that, Cairo merely supplied more mana into his skin to withstand the dissolving power of the black fog. If he doesn't do that then even his skin will start to melt and that would be another injury on his part
The space was already filled with sand balls that were destroying chunks and chunks of the mage's body. Cairo only needs to hold out until all those energies were spent