He would have strapped a bomb to his body. Or some other means of horrific terror. Cho was help bent on murdering lots of people. Lack of a gun would not have changed that.
It takes a lot more resourcefulness to get a body bomb than to amble across town and buy a gun using your Master Card.
Regarding using a chainsaw, its hard to kill 32 people in 60 seconds using a chainsaw.
(Obvious points, why should it be necessary to even make them.)