Command & Conquer: Red Alert 3

One of the easier skirmish achievements.
Play in a skirmish vs a medium AI.
You need to be Rising Sun, pick any enemy AI, although I used Soviets with Oleg [he tends to rely on infantry].

Play on the 2vs2 map called Snow Plough.
As soon as the game starts, you want to be quick with building, so you can get an upper hand, build 3 Dojos [try to keep one queued at any time], 2 Power cores and 2 Refineries.

As soon as the dojos are complete, build 20 imperial warriors and 10 tank busters. Once you have a decent number of units, rush the enemy with them, select the unit types by double pressing A to select them. Using this, select all the imperial warriors, and when you get near a bunch of infantry units, press RB+X to use special ability. You'll see them pull out beam katanas and charge the enemy. Shouldn't be too hard to get 15 kills this way. If it doesn't look like you'll get it in one go, you might want to go back and build a second army to get it.

This tactic relies on getting to the enemy base before they have a lot of vehicles, if you get there early enough, you can even wipe out their construction yard and war factory, and then they'll just produce infantry for you to run down.