Well you don't have to use collisions. You can pick the closest object within her angle(granny.x,granny.y,zombie.x,zombie.y).
With that many objects running at you you shouldn't have to worry about accuracy.
All people want to see is something blow up.
Also try to scale down the physical size to the nearest power of two size. You can then up-scale the images in game.
A lot of games do this, and yes some of them look like crap.