What you do.
What I did, to get my 10 light balls, was went to an empty Box, went to the Safari Zone. Saved before I went in. The patch of grass I ran in. Was just soutwest of where you enter. Go South til you stop, then west, first bit of grass.
Then, just throw a ball everytime you see a Pikachu, checked the box once I was done. If none had any I restarted, and did it again. Most often I would just keep catching, had 35 Pikachu at one time. lol.