It is surprisingly difficult to stick a grenade since these fellas move quite a bit.
The easiest way in my opinion is to use Lift on an enemy and then pop the grenade on him. Works like a charm.
Make sure to stick Outsiders only (the humanoid aliens) since all others don't count for this. You can stick all sorts of Outsiders though. Commanders, Phantoms, standard ones...doesn't matter as long as it is an Outsider.