The Last Water Bender

The art of war is to flip your opponent's strengths.
They are large, but slow.
They are strong, yet weak where armour is chinked. Their supply lines become your supplies.
Their armouries become your weapons.

The intel is good. They will attempt to breach your defences by sea at 04:30 before sunrise.
You know for certain that the attack will come from the ocean since they've destroyed all bridges in or out of your few remaining strongholds.
The mountainous terrain renders their infantry units ineffectual against guerilla tactics, and natural dead-zone pockets of radio interference created by metals in the surrounding cliffs have made easy targets of their air attacks.
A defeat here would signal the beginning of the end of the resistance while victory will be a tipping point as winter is fast approaching and their reserves are running low.

They can only come from the beach.

In the first days of the invasion, before the looting could even begin, you and the boys had already determined which places to infiltrate and defend while leaving common grocery stores and the like to the looters.
Many of the goods hauled to safer locations seemed unnecessary at the time. For instance a mis-shippment of 10 pallets of Seal-Skin shoe spray being returned rather than the 10 bottles ordered by the local Five'n'Dime may very well be our salvation.

Weeks have been spent creating the infrastructure of your trap.
More than 130 dump truck loads of dry beach sand has been made to be hydrophobic.
Massive sand bag dams hastily erected in the dead of night while the low tide window lasted allowed the few remaining pieces of still functional equipment to replace a vast section of the beachfront with dry hydrophobic sand but not before laying perforated tubing beneath each pit.

The enemy has waited til highest tide to reach as far inland as possible before disembarking and as the landing craft breach the surf the men and bots begin to pour forth.
They meet no resistance as they begin to arm drones and unpack larger pieces of equipment.
The signal is given and huge volumes of compressed air erupt from the buried airlines rendering the solid appearing beach to become as liquid as the water they emerged from.
Previously floating vessels remain afloat but more than 90% of the men and equipment instantly sink more than twelve feet deep at which point the air is disengaged locking all and sundry within the still dry sand.

What remains of the invading force surrenders with minimal persuasion and will be ransomed for your own POW's.

You withdraw beyond range while their few remaining ships wastefully deplete precious ammunition and, when they finally withdraw, the air lines once again fluidize the sand as massive cargo nets raise your newly acquired weapons and supplies to the surface.

You will put them to good use.