Short conduit run from attic to exterior panel. What is the correct way to run cable?
If you can't get the cable down the wall cavity, then use a junction box to transition to THHN in conduit and go down the wall that way from there

Normally, this run would be inside the wall cavity and then enter the panel from the back -- even if it's somewhat harder to run that way, it has the advantage that you don't have to mess with conduit.  If that's not possible, though, the alternative would be to run the NM into a box in the attic, where you transition to THHN in conduit (1/2" EMT or PVC suffices for this) and then run a short length of conduit out the wall into a LB conduit body, then conduit down the wall to a point alongside the panel, where it uses a LL or LR (depends which side of the panel you're on; if you can't be sure when you're buying it, simply pick up an Arlington AnyBODY or equivalent configurable conduit body instead of the LL or LR) body to transition into a nipple into the side of the box -- do not enter the panel from the top! (NEMA 3R boxes require a hub fitting to enter from the top, and generally only have knockouts for a single hub atop the box.)