Dark Void

You'll first see a Knight after the village event. You can get this achievement right here during this fight. It's like a boss fight, but you'll see many more by the time you get to the end of Episode 1. There is an ammo restock kit on the platform you're standing on when you fight this Knight, so if you use up your grenades, be sure to grab the ammo kit. The Knight is really easy on Hardcore Mode, so I assume it's even easier on the lesser difficulties.

What you want to do here is to attack the Knight with your rifle (I prefer the ones the Watchers drop). Once you start seeing something that looks like a blue flame around the Knights head, cook your grenade a bit, toss it at the Knight and pop the achievement.

This is likely the easiet place to get this achievement as there are no other enemies, just the Knight. But you will have plenty of other opportunities later on. I try to nail them all with grenades because I am hoping it counts towards https://www.trueachievements.com/a41528/fireworks-achie....htm