Seems hard, but it's quite easy. It is much easier to do this solo, but it is highly likely that you will get this without even trying during your travels. If you are unable to get it. Play the very first mission of the game in solo mode. Run around and get as many enemies as possible to follow you. Once they are all following, throw the grenade and it should kill far more than 20.
Kill 20 or more Xenos with a single mine or grenade