Requirement: Win a battle as Guan Yu, then save your game after the battle.
Walkthrough:
- Select Empire Mode. Put it on Easy mode to speed things up.
- Select Guan Yu and choose your level. You'll want to pick a level where your character is not a Ruler because Mercenary Missions are only available to Officers. If you're having trouble, the A Land in Chaos scenario lets you pick whether you're a Ruler or Officer no matter what character you choose.
- Once you're at the Empire map screen, select Battle, then Mercenary, then Flame Tigers. In this battle you just need to defeat the six "officer" Flame Tigers. Find one on your map, kill the one with a name over their head, wait for a moment, kill the second tiger that spawns in the area, wait again, and then kill the larger third tiger that finally spawns. Continue to the second set of tigers and do the same thing to end the mission.
- Save your game by selecting System, then Save at the Empire mode map menu, and the achievement should unlock.
You can use this same method to unlock every character achievement in this game.