You receive a "Here, Hold This" Medal for killing an enemy with a thrown back grenade in TDM (including 3TDM, FFA, and Classic) and Objective games.
However, it's also possible to get these Medals playing Co-op Hunter Arena - but only when playing as a Villain. As such, the most convenient way to get these Medals is whilst performing Hunter boosting for Treasures.
Villain should run to the Heroes and place their aiming dot a little beneath the centre of the line between the two Heroes' feet. One Hero should throw a grenade at the feet of the Villain, then the Villian throws the grenade back (using a timed Triangle button press). This should result in two "Here, Hold This" Medals; if performed efficiently, the trophy shouldn't take more than two Hunter games.
Once again: it's only possible to get these Medals in Hunter when playing as a Villain.