DOOM 3

In Erebus level 3 right after you get the Super Shotgun and the second power cell as you're leaving that area your screen will turn red and you'll be swarmed by a bunch of zombies. Just activate Hell Time, switch to your grenades and start chucking them. It doesn't seem like you even need to kill 5 with one grenade as I seemed to only kill 4 with my first grenade and 2 with my second and that's when the achievement unlocked, so as long as you kill 5 during one Hell Time the achievement should unlock. Actually if that's the case then you could in theory just use the regular shotgun to blast the zombies, especially if you're playing on something higher than recruit.

If you're worried about not getting it the first go around just make a save when you grab the power cell that you can reload and try again with just in case.