12,000 Burmese vs. 30,000 Chinese Imperial Army

The story you’re looking for is likely the Third Invasion of the Sino-Burmese War (1767–1768), specifically the Battle of Maymyo. It is one of the most incredible "underdog" victories in military history.

The Conflict: David vs. Goliath
The Qing Dynasty of China, at the height of its power under the Qianlong Emperor, sent a massive force to crush the Konbaung Dynasty of Burma.

The Chinese Force: Led by the Emperor’s nephew, Mingrui, an army of about 30,000 to 50,000 elite Manchu Bannermen and Green Standard troops marched toward the Burmese capital, Ava.

The Burmese Force: The main Burmese army was actually away fighting in Siam (Thailand). The local defense force was significantly smaller, around 12,000 men (including 10,000 infantry and 2,000 cavalry).

The "Death Trap" Strategy
The Burmese commander, Maha Thiha Thura, knew he couldn't win a head-on collision against such numbers. Instead, he used the terrain as a weapon:

The Bait: The Burmese allowed the Chinese to advance deep into their territory, stretching the Chinese supply lines thin through the dense, malaria-ridden jungles.

The Pincer: Once the Chinese were exhausted and hungry, the Burmese veteran troops returned from the Siamese front. They executed a massive pincer movement, cutting off the Chinese retreat back to Yunnan.

The Encirclement: At Maymyo (modern-day Pyin Oo Lwin), the Burmese completely surrounded the Chinese main army. For three days, they hammered the trapped imperial troops.

The Aftermath
The result was a total catastrophe for the Qing. Out of the tens of thousands of Chinese soldiers, only a few thousand managed to escape. The rest were killed or captured.

The ultimate drama: General Mingrui, realizing he had lost the Emperor's "invincible" army, chose to cut off his own ponytail (the queue), sent it back to the Emperor as a sign of loyalty, and then took his own life.

This war is why Burma (Myanmar) remained independent and was never absorbed into the Chinese Empire.