Firstly, if you're unaware what the "Hedgehog" is, then it's the grenade of which once thrown, it launches spikes out in every direction, and looks like this.
A somewhat easy way to obtain this trophy would be to acquire yourself a hedgehog and hold onto it, saving it until you come across a group of chimera (the weaker soldier chimeras, the ones with blackish armour don't die from the hedgehog first try), it would be a lot easier if there was a checkpoint recently.
Now, throw the hedgehog at said weak group and get yourself a bunch of kills, once done, if it's possible in any sort of way, get yourself killed so you can respawn with the grenade and group of chimeras once again. Dying does not reset your kills, so you can easily grind it out in many locations.