How to run cable from attic to breaker panel in a finished room
If the wall is an external wall this is really tough.
 I have used fish rods (thin fiberglass rods pushed through the panel.
  One advantage today that make it possible are clamps like raco insiders these Romex clamps can be put in after the wire is pulled from the inside of the box. 
(Many home owners just left the cable in the opening without a clamp). In 2 cases I have been called to repair when the hot shorted out on the sharp edge of the panel that is not a lot but one required a new panel and a lot of wiring to repair burnt wires, the other just re-pulling the burnt wire.

I have found putting a pull string on the leading edge of the fish rod and going up until I find the hole in the top plate works best.
 Remove the string pull the rod out through the bottom , tie the string to the Romex and pull the cable down to the panel. 
2 people make this much easier For me I would almost rather repair a small section of drywall and have to paint than fight getting a fish rod through the panel and then the top plate by myself.  the “insiders”  plastic clamp makes this possible but still a chore. I usually put a hard plastic shield over the lugs if the panel is a main lug style and am very cautious when trying to find the hole above not to come in contact with any hot wires. hope this helps.