You ask your project team what they think by when the software can be delivered, based on a sustainable work pace. Ask them to make that estimate challenging but reasonable. Give that estimate to your manager. Leave it at that.
If your manager „demands“ that you work faster, ask for a reduction in the scope of the task or otherwise indicate that quality will have to suffer.The idea that software developers can magically work faster without sacrificing quality or scope is one of the last great myths among managers who shouldn‘t be manageing software development.
If your manager tells you that schedule, scope and quality are fixed, just shrug and start working without putting in unsustainable overtime. What do you think will happen if your team exceeds the deadline by, say, two weeks? Will they all get fired? Or will your manager see it as the typical delay that you have in every software project?