Tomb Raider: Underworld

In the last level when you are in the tunnels towards the final room, lots of enemies come charging you.
Make a grenade stick to one of them and wait for it to explode.

A quick way of doing this is whenever you encounter two enemies close together (the ship on the first level works), throw a sticky grenade on one and shoot it, the explosion should wipe out both guys.

In Southern Mexico (Land of the Dead) towards the end you will have to fight of some kind of ghosts. They spawn and if you don't kick them when shot they will revive.

Easiest way to get this achievement here is to stick a grenade to one of them. They will run both towards you and the grenade will kill them both. On the plus side you also don't need to kick them anymore with Y :).

See the video around minute 2:00 which creeps I mean and where..

I did this alongside this achievement: Soon after you start Level 3: Coastal Thailand - Bhogavati, you'll come to an area where a cutscene pans to the left. As you go down some stairs (one set on the left, one set on the right, it doesn't matter which), five komodo dragon-like enemies will spawn. Take the health of these down a bit, and then throw the grenade into the middle of them, as they should all be trying to crowd around you. The grenade will explode, giving you the achievement.

Also, if you don't care about achievements not having a date stamp (i.e. being earnt offline), remove the patch by clearing your system's cache, and then when loading this area, hold LT, and whilst holding LT, press A, Right Trigger, Y, Right Trigger, X, Left Bumper.

This will give you invincibility, and lets you stand in the middle of the area with the five komodo dragon-like enemies surrounding you. Use your pistols to lower some of their health, and then chuck the grenade at your feet. It will hit one of them, and pretty much guarantee you the achievement.

Hope this helps.