Requirement:
- Defeat Da Ji within 1 minute of her betrayal.
Walkthrough:
- In story mode, select Wei Mission 7 on easy. The Impulse ability will come in handy here to increase your speed. Increasing your damage can also help, but isn't all that needed for this one. This is the easiest 8-X to unlock.
- At the beginning of the mission head to Zhuge Liang on the mountain to your north and defeat him to capture the mountain and its cannons. As soon as you defeat him, pause the game and locate Da Ji (she's the yellow icon). She's probably to the south near the middle.
- Head straight for Da Ji as fast as possible. As soon as all the dialogue caused by you capturing the mountain ends, Da Ji will betray you. You'll know when this has happened when she says "bye" and turns red. From the moment this happens you have one minute to defeat her.
- Once Da Ji has been defeated, head to Dong Zhuo (north east corner) and defeat him to complete the mission. Save your progress and the achievement should unlock.