If you are willing to get the 5th weapons but don't know how, make sure you have the level on very hard these are the people who I know:

Lu Bu
Stage:Chi Bi
Method:Make your morale big then i think
Dong Zhuo will send back-up troops from the level at chang ban defeat them, special item report.

Diao Chan
Stage:Diao Chans 1000 journey (new stage?)
Method:Defeat Lu Bu's back up troops and the general himself, special item report.

Yuan Shao
Stage: Hu Lao Gate
Method: Defeat Diao Chan, Lu Bu will get pissed off at you special item report.