Total War: THREE KINGDOMS

You can probably get this in 3 turns with Tao Qian (free DLC). He starts literally next door to Cao Cao, who starts with Xiahou Dun in his army. When you start, follow the story until you take Pengcheng, Pengcheng. Unlike most factions' starts, you won't have the movement to get there, so this will take you 2 turns at least. On turn 2, Cao Cao will usually take Chen, Peixian, putting him in striking distance. If this is the case, attack him immediately on your next turn. Start the battle then order your archers to attack Xiahou Dun directly. The achievement should pop during the battle soon after he gets hit with arrows. Even if Cao Cao doesn't attack Chen, Peixian, his starting land of Chen is still very close, so just move around a bit and look for him. If you siege his land, he might react to that and come to you as well.