The fifth campaign in Story Mode is too difficult. How do I clear it?

[Dot76(KR)]

The fourth campaign was manageable, but the difficulty spikes drastically in the fifth.
For example, just when I barely manage to exceed 300 population, the game suddenly requires me to build a structure that demands a population of 600.
It also asks me to send 20 military units, which feels like an unreasonable requirement...

I tried four times today over the span of 12 hours, but I failed every time.
How can I clear this stage?