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?
315 completed games to 100%
91.02% completion percentage
Last Achievement : 100 Times Too Many - Alien Isolation (Xbox One) - 5g
Last 100% Completion : Alien Isolation (1000g) (Xbox One)
Thanks to Rajeanero for the avatar and Audilicious for the sig.