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?
365 games played
226 completed games to 100%
89.07% completion percentage
Last Achievement : Gotham Protector (Flying Solo in Trine 2 for 15g)
Last 100% : Trine 2