Lara Croft and the Guardian of Light

This achievement is basically impossible not to get. When you're attacked by enemies, there's usually 5-6 of them at once, and they like coming at you all clumped up. This can be done on all enemy types, simply drop a bomb with , wait for the desired enemies to get within explode radius, then press again. *Achievement Unlocked*

Though if you need a specific location, there's a level called 'Spider Tomb' (It's the 3rd stage from the level select screen), which has you starting off on top of some stairs. Descend these stairs and you'll see a huge pit ahead. Drop a bomb just between the stairs and the pit, walk slowly forward, and you'll trigger a whole bunch of spiders. Now simply let them get within bomb range and *BOOM*.

This can be done on pretty much every level, and you should get it by Spider Tomb very easily. You can get up to 10 kills with the Remote Bomb.

Thanks to Achievaholics

When you start coop mode even for 1 player, the game spawns more enemies at once. This can be done several places on the first level. If you do it in single player, its harder on first level as there isnt many enemies at once.

I coop, you can use the second player as a bait in the bomb circle btw. to quickly attract 3+ monsters inside the ring.