A drone swarm dropping hand grenades is like targeted artillery. Experience from WW 1 is that it's straightforward to defend against artillery fire, just retreat into a shelter. A target that's worth five dozen drones with grenades can afford a bunker.
So simple in your scenario... The target just needs to jump in their nearest bunker, better hope they happen to be attacked outside their bunker eh?
Not sure why you're being downvoted. Legitimate concern.