Yes you can, there is the same chance to encounter shiny Virizion as any other shiny Pokemon. (1/8192)
EDIT: In order to better find one, just save right in front of it and keep battling it and soft resetting until it is shiny. This will take quite some time though, so prepare to get really bored. I would also suggest getting the shiny charm first, as this will significantly increase your chances of finding one.