In 1592, the second "Unifier" of Japan, Hideyoshi Toyotomi, launched a massive invasion of Korea, with modern estimates putting the Japanese invasion force at 250,000. The Koreans were disorganized and ill-equipped; the Japanese had been using Portuguese models of early arquebuisers, and many of the commanding samurai were veterans of the sengoku jidai. The Korean regular army was shattered in the following months, and Japanese troops actually reached as far as the Yalu River before the Chinese intervened. To an extent the Japanese had come prepared with better technological advances in the contemporary military field; however by 1594 the Chinese intervened and were supplying a lot of gunpowder-weapons and canons to the Koreans (Crouching Tigers, Secret Weapons: Military Technology Employed During the Sino-Japanese-Korean War, 1592-1598 is a good source for the military technology used on each side)

After the major victories secured in Korea came to a halt, two separate 'guerilla-style' wars were fought by Koreans. In the sea, the now legendary Korean Admiral Yi Sun-sin began raiding and attacking the Tsushima straits, inflicted heavy losses on the less-prepared Japanese navy. This type of 'behind-the-lines' attacks on Japanese supply routes at sea played a pivotal role in securing a Japanese withdrawal in 1598. The Koreans were able to use the familiarity of the coast lines, tides, currents, and obstacles to their advantage over the ill-prepared Japanese.

On land, after the taking of Seoul by Japanese forces, guerilla bands of Korean troops began popping up around the peninsula. Guerillas harassed Japanese supply lines from Pusan, using hit and run tactics, and by the end of the first year the Koreans were able to push the Japanese out of most of the towns in the surrounding countryside. Quite often, local guerilla units would merge with reorganized Korean regulars, and retake strategic forts deep in Japanese-occupied Korea. The Battle of Haengju is an example, with 2,800 Koreans defending numerous attacks against a much larger Japanese force. The battle is extremely symbolic in modern Korea.

The guerilla forces were able to stall Japanese troops long enough in order for China (Ming Dynasty) to enter the war on Korea's side. Although after 1594 the Korean army generally reorganized itself with the help of the Ming, the service of the guerilla forces that organized themselves still remains a large part of modern Korean nationalist identity.

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I'm not sure exactly if this is what you were looking for, but it is an isolated incident of guerilla tactics and militias being raised to fight an invading force that had overrun the regular army. I also am by no means an expert on Korean history, though this particular war does interest me, so there's probably some stuff missing, but this is the basic gist of it.

Some sources touching on the subject are:

Steven Turnbull, The Samurai Invasion of Korea, 1592-98

https://www.ancient.eu/article/1398/the-japanese-invasion-of-korea-1592-8-ce/

https://www2.hawaii.edu/~sford/research/turtle/ - this was an interesting little link.