The quickest way this can be done is on the first mission on arcade or story mode.
Use the homing missiles (the weapon that locks within the circular area on and shoots a laser at the enemy)
At the start of the mission, before any enemies appear, go to the bottom right hand side of the screen and use the homing laser.
This should tag a dog and unlock an achievement.
Friday, September 16, 2011 @ 12:23 PM
Monday, September 19, 2011 @ 05:50 PM
Monday, December 12, 2011 @ 01:21 PM
Sunday, January 22, 2012 @ 07:42 PM