Alien Breed: Impact

This can be done at the start of Level 2 along with the "Massacre" trophy. 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 having trouble, see "Massacre" for an alternate location/method.

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.