Bend the Conduit
A conduit is a metal pipe that houses electrical wires. When wires are tightly wound through a conduit, a grounding wire isn't necessary -- as the conduit itself acts as a ground. (It's important to use conduit when running wires on the outside of a wall.)
Conduit benders will bend the metal of a conduit, or a 90 degree pulling 'L' can be used (image 1). It opens in the back, which creates an access point to pull wires through. Run the wires from the subpanel (image 2) to the pulling 'L' and then down to the junction box (image 3).