For this achievement, you need to use a Team Skill and get at least one kill from it. Team Skills will show up randomly as you level up in a match or if you have an item that unlocks the skill as soon as the match starts.
If you have a team skill, press to begin it. Several circles will now show up on the screen. Other players on your team need to fill these circles. Press again to launch the skill once enough team members fill the circles.
Skills that can be used for this are the Warrior's Group Rush, and the Mage's Lightning Arrows, Tornado, and Meteor Shower.