How can I attach pony wall to surface of platform and will it be strong enough against lateral forces?
I'd build the wall in such a way that the entire thing acts as a post: 


Use 3-1/2" or 4" construction screws instead of nails at all joints
Use construction adhesive at all joints
Use screws and adhesive to mount the bottom plate to the floor framing


This configuration will be as stiff and strong as it would be if you used occasional steel bracing. Any right angles or other wall connections will enhance strength.