How was the PLA, still on the heels of fighting a civil war,

Zerg rush

>>2017705
the PLA took pretty horrendous casualties.

>>2017708
>>2017711
These pretty much. Sheer numbers is all there was to it.

>>2017705
surprise and outnumbering their enemy 20:1

Numerical superiority is a helluva drug

http://bevinalexander.com/excerpts/korean-war/tactics-chinese-communists-korean-war.htm
They fought with experienced troops who have been fighting for over a decade in both the Sino-Japanese War and the Chinese Civil War soon after. Their experience is reflected with superior light infantry tactics against superior firepower similar to the Sino-Japanese War and the Chinese Civil War along with their experience in guerilla warfare.