I did it on high noon, elite slayer, guy had no shields, stuck me, then died, “You killed verylonggamertag”. No achievement. I killed him with his own sticky (i think) might have been stuck from another guy but unlikely as i saw him throw the grenade.
He needs to kill himself.
It should say suicide for him.
“Kill an enemy with their own Plasma Grenade that is stuck to you in a matchmade game”
All you have to do is get stuck, run up to who stuck you and do a kamikaze.
Seems like you’ve done what you were told… try avoiding hitting him with bullets, that way you don’t get the point.