From Halopedia, the Halo wiki
Walk-Off Winner is an Achievement in Halo Wars that is unlocked when the player uses one of the 6 Major Leader Powers to destroy an enemy's last unit.
It is worth 30 Gamerscore.
Go on the advanced tutorial, then, when you are in control of Regret, move the Shua'ee-pattern Locusts away and then kill all of the enemy units outside of the UNSC base.
Play on Release and release the Flood, and then use a leader power to destroy them all.
Play a 1 vs 1 with a friend and get him to buy no units, then use a leader power on his scout unit.
The achievement can also be earned on the campaign level "Scarab" by killing the scarab with a MAC blast.
- The name is a reference to a "walk-off home run" in baseball, in which a batter hits a home run in the bottom of the ninth inning or later, and in doing so adds enough runs to their score to win the game. As a result, both teams "walk off" the field once the winning run is scored.
- Even though the title says an "Enemy's last unit" you can actually get it by killing an ally's last unit with Regret's cleansing beam.