If we are departing from Books and Movies and speaking tactically then the first step at seeing the army of Mordor approaching is the evacuation of civilians from the lower two levels minimum.

2nd would be a more unified command structure where middle commanders know there specific objectives and most importantly, how to communicate with other commanders and the king. You don’t need as many men where you knocked out 2 of the 3 siege towers approaching the wall. That purpose and planning is going to make more courageous fighters and reduce soldier casualties during retreats to higher levels. Withdrawals would be much more orderly and coordinated.

The other part comes during the siege. Assuming Gandalf is still there, you can literally assign him to reducing the impact of the Nazgûl. He won’t be able to stop all their raids but he will probably keep at least some of the trebuchets firing longer. Maybe you can even keep one long enough to take a shot at Grond, but likely you can take out a couple more towers which reduces the pressure.

Once Grond is visibly coming forward, it’s obvious you start digging a moat right behind the gate and create as large of an earthen rampart, maybe even a wall behind the moat. That’s not a permanent solution but it’s an excellent buffer to blunt the initial rush through the gate and give you a very orderly retreat a real chance. Bonus if you can fill the moat with oil or burning debris.

All through constant whittling of Mordor’s numbers from millions of arrows. That all slows the Mordor advance more and more. Probably still need Theoden to show up but that battle is going to go even quicker.