The most reliable way to get this guaranteed is on the first encounter with an Avatar.
Spoiler tags needed.
*** Spoiler - click to reveal ***You will meet your first avatar when you skulljack a codex. The trick is to bring 1-3 sparks on this mission and make sure they get the killing blow. The achievement pops when the Avatar dies.
The second opportunity to meet Avatar's is in (double spoiler here!!!) on the last mission. There are only 4 Avatars in the game so you need to kill an Avatar on one of these two occasions.
I would recommend right before you attempt this achievement changing the difficulty before launching the mission to rookie so you can ensure your SPARK gets the killing blow. I would recommend the first encounter as you can kill off all other enemies first if needed or at the very least there is less chance of death.
The Avatar teleports after being hit so if you can arrange it so you're in an open area that is ideal.