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?
283 completed games to 100%
90.42% completion percentage
Last Achievement : Weed Whacker Plants vs. Zombies Garden Warfare (Xbox One) - 50g
Last 100% Completion : Plants vs. Zombies Garden Warfare (Xbox One)
Thanks to Rajeanero for the avatar and Fishman for the sig.