Alien Breed: Impact

This can be done at the start of Level 2 along with the "Electrifying" trophy. Despite the misleading description, this trophy can be achieved using a frag grenade. Probably easiest in the second area that the physician opens up for you. You should run in and throw a stun grenade into the group of small aliens. You can try throwing a few into the same area if you don't stun enough the first time. When you have a large group of the small aliens stunned together (10+), throw a frag into the middle and hope that it kills 10. If you don't manage it first time, you can just reload the save from the start of the level and try again.

If you're finding this trophy hard or didn't get it at this location while playing through the story, here is another location that might is better if going for the trophy through Free Play (thanks to Dark_Phantom_89 for this):

I got Massacre right at the beginning of Level 4 after realising that doing Level 2 on Chapter Select doesn't give you a frag grenade.

I simply stepped out of the elevator and was rushed by around 15 small alien enemies. I ran around for a bit to get them clumped together, stunned them then got right in the middle and just dropped a frag grenade - I didn't hold L2, I only pressed it and Conrad literally dropped the grenade and it instantly exploded. If you mess up, you can simply quit the game, load up the chapter again in free play abd try again.

Although the trophy description says "(or witness another player doing this)," some people have mentioned that while playing co-op when their partner achieved this trophy it didn't unlock for them. For the people that have said this, the other person who achieved the trophy was the host.