Xeno Crisis

You will most likely get this achievement naturally playing through the campaign, especially with the debug menu which gives you unlimited grenades.

If you want to get this quick you can get this very easily at the beginning of the level The Facility. I personally used Seed 1.

First of all, you will want to bring up the debug menu. On the main menu highlight Options and then hold down , , , and then . You will want to change the level to The Facility. Also pick Seed 1 to get the same map layout as me. You can turn on infinite ammo and infinite health but it is not necessary since you can get this achievement in the first room of the level.

Next you will need to bring up the Backer menu. Go back to the main menu and highlight Options again and then hold down , , , and then and choose backer 7 Jacqui. Also select The Facility as the starting area.

You can also change your difficulty to easy on the options menu.

At the very beginning of the level you will spawn in a room with 6 enemies. Wait a second for the enemies to bunch up and then throw a grenades directly at them so that when the grenade touches an enemy it will instantly explode. In the even that you could not kill 5 enemies with that grenade immediately turn your attention to the top left of the room where 5 enemies will come charging out of the door at the same time. Throw a grande at the charging enemies for a direct hit of your grenade.

For a visual guide you can watch my S Rank run through of the facility where I kill 6 enemies with a single grenade at the 7 second marker.