Overseers are the enemies who like to use grenades. The first time one is encountered is when entering Holger Square in the High Overseer Campbell mission. He is bullying overseer Martin and is alone. After he sees you, run away to get to a distance where he will throw grenades. Use Bend Time to pick up the grenade and throw it back. This may need to be repeated a few times until you get the kill. Have some health and mana potions available, especially if you need to use Bend Time multiple times.