Play the level as the Han forces under free mode and the hard difficulty setting with a character who you want to make stronger. From where ever you start, go straight to the allied Gate in the bottom left corner and defeat the general who is attacking it. Then, go to the river where there is a scene of the it being frozen, Immediately run back to the gate earlier, without killing any more generals. Wait until Liu Bei, Sun Jian, and Cao Cao die, leaving the castle under siege. Wait until He Jin is under attack and help him. All that remains is to kill anyone who gets in your way. The Gate closest to Zhang Jiao is the only one that will send units your way. Getting a horse to go and close all the Gates is helpful. In doing this, you should be able to kill over 1000 to 2000 enemies, raising you five or six levels.