Two things :
I got spreading the love after using volley. I hit all five enemies. Therefore, grenades and firebombs aren't needed.
From this I can also assume that hots like buckshot, or any that display a cone, will work.
CAn anyone confirm quitting the mp matches unlocks the achievement?
369 completed games & apps to 100%
91.47% completion percentage
Last 100% Completion : Batman Arkham Knight (1000g) (Xbox One)