Manipulating the game settings is the key to getting every achievement in NHL2k6 easily. These settings can be found after you choose your team in quick play mode. Press the Y button at the settings screen to access them and the bumpers to scroll through the different settings.
For this achievement, set the difficulty to Pro or higher, set all of the AI skill sliders down to 0 and all of the Human sliders up as far as they will go. It is also best to set the period clock to a full 20 minutes so you have lots of time to get the goal. You shouldn't need more than two minutes, but it's best to give yourself plenty of time. It's also best to select a very good team for yourself and a very bad team for your opponent. Denmark seems to be the favorite team to beat up on.
Now, it's simply up to you to score a goal with a defenseman. It should be very easy with the sliders adjusted, but if you screw up, just restart and try again. This achievement will stack with others completed at the same time.
As others have suggested, you can also use a second controller to pull the opponents goalie. This is a surefire way to get the achievement if you have a second controller. Just make sure it's a defenseman who scores the goal.