Trigger the rain manually instead of wait for it to happen automatically.
Defeat 9 of the 10 enemy generals. The rain will fall automatically approximately 5 minutes into the stage. To trigger the rain manually, achieve 26 KO Count at the beginning mine field area of the game in under 5 minites. The 10 enemy generals of this stage are Zhuge Liang, Wei Yan, Gao Xiang, Wang Ping, Yue Ying, Liao Hua, Zhang Yi, Ma Dai, Jian Yong, and Jiang Wei. Defeat any 9 of them. Wang Ping's ambush troops will appear after you go up the map right path. Yue Ying will appear after you deplete Zhuge Liang's HP to approximately less than half. If Sima Yi reaches the end of the map top right path, he will successfully escape from the battlefield and end the stage. Do not make the mistake of doing so before you get the weapon, like I accidentally did the first time I played the stage, which was also the first time I tried to unlock this weapon. This must be done in Expert
Get 800 kills in Hard Mode
On hard mode get 50 kills in 4 minutes
Get 100 kills in 3 minutes on hard mode
Achieve 300 KO Count in under 4 minutes on hard.
Get 100 kills in 6 minutes and 35 seconds on hard mode
Get 300 KO Count in under five minutes and win with the difficulty on hard
Achieve 100 KO Count in under 4:30 minutes on hard.
Don't let any one pass the bridge and kill all generals on hard
Kill Cao Ren, Zhang He, Xu Zhu and Xu Huang on hard mode
On hard mode, get 400 KO Count before Xiahou Duns reinforcements comes
Before your ally troops get eliminated, defeat Guan Xing and stop the rock attack on hard
Beat Hua Xin, Hua Xiong and Lu Bu in under five minutes on hard