Pretty easy, when an enemy is low on health, just tap LB to request fire on them. Assuming an ally was looking that way, and hits, you get an achievement! 30 seconds is a pretty long time in game, so you could even just tap LB as soon as an enemy is spotted if you have good support, since it may die in that time anyway.
P.S. If you try to get it in artillery, you must NOT be in bird's eye view when you request fire.