Defeat an opponent player on the same panel as you using Ambush.
This is easy enough to manage with high attack characters such as Yuki, Tomomo, or Store Manager, or characters with strong battle hypers like Peat, Syura, or Nanako. Luring an aggressive CPU by walking in front of them or being at low hp will put them on the same tile as you, and as long as you can survive the first battle, the second should be a cinch, since now you get to attack first and they're probably already damaged from the previous fight. Luring players in multiplayer by getting a solid star lead makes you a high-priority target and again easy to manipulate into Ambush.
The requirements for this achievement were changed in Version 1.10. The change allowed for KOing one opponent with a single Ambush instead of two opponents, decreasing the requirements for players who had not previously completed them.