Ka-boom! is an achievement/trophy, awarded for killing 10 enemies in Metro 2033, and 30 enemies in Metro 2033 Redux, with either normal or sticky grenades. This is not very difficult to earn as grenades can be frequently found on fallen enemies or traps and they can kill fairly easily.
An easy way to gain a large number of kills, at the start of Frontline, as the soviet commander is giving his speech, throw a grenade at them, this will rack up alot of kills quickly. Farming the nosalis waves in Dark Star is another easy way of getting this achievement, since they are all bottlenecked in the tunnel and you can just use up both your regular and sticky grenades. Once you run out, you can just reload the last checkpoint until you get the achievement.