Finesse To Spare.

Tandrak just had to make some modifications and increase his energy input, the shield began to weaken in no time. The solid translucent surface of the barrier became transparent and waves began to spread across its surface as it tried to resist the intrusion.

The waves were created inwards towards the point of weakening. The shield was a creation of the meshing of barriers by different sources. It can resist physical and magical attacks due to its special energy structure. Tandrak was siphoning the energy of the barrier causing it to lose its strength. Trying to overload it with his energy will not work. He doesn't have as much energy as the combined power source of the war fortresses. Plus it will be a waste to do so. Why not take energy from them instead? The same energy that they need to power the barrier.