Well, if you managed to detect and track an incoming drone you could ‘jam’ its camera with a high-powered laser - or something that threw up a curtain of flares or burning magnesium, or whatever.
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.
