I got it first while playing Zhang Jiao story mode. Took me 30-45 mins. Then I improve my conditions and tried with Diao Chan. Took around 15 to 30mins. It works very well for me! Share this for fun.
Go to Zhang Jiao Story Battle or Free mode with any characters choosing that stage. Follow the flow of battle until you defeated Zhao Yun at the top right corner.
Then you just need to camp at the right side of the map(where there is a narrow stair going up to a small area), that quite a good peon spawn area. Not sure is it unlimited spawn anot.
Equip 2nd, 4th, 6th and 7th skills(from left to right, top to bottom), and just musou those peons to fill awakening bar, and keep doing A.Musou around the stair and the area(DONT GET PASS THE GATE) till you get 9999KO. Try to get musou pots and seal to keep the Musou bar from finishing and your A.Musou should last for a long time...
If you did get pass the gate, or if the officer guarding that area did find you, kill him, kill CaoCao and LiuBei, then went back to the area and continue the spawn kill.
By that time, you would have level quite a lot from the kill and the +5 scrolls. Impressive chains too. :)