How do I level high-rank units quickly?
Solution 1:
Unit experience is calculated based on an "internal level", and promoting a unit instantly adds 20 levels to this counter (making them effectively level 31 at minimum).
This winds up meaning that promoted units get significantly less XP for killing non-promoted units.
So no, if you want to pass on Galeforce to Lucina (I'm assuming?), you will need to grind out those levels the hard way. (Reeking boxes help)
That said, keep your eyes peeled for the Entombed -- these yellow Zombies are worth bonus XP (as if they were a promoted unit, I believe), and most risen encounters (including those from the Risen box) will have exactly one.
Alternatively, you could simply not pass down Galeforce -- as long as Sumia is her mother, Lucina will be able to reach the Dark Flier class herself -- she won't start with it, but she'll be able to end up with it in due time all the same.
Solution 2:
Remember to use your Reeking Boxes on the location that you've cleared most recently; this way the enemies will have the highest possible levels.
Other than that, the best way to gain experience quickly is to fight the highest level enemies that you can beat.
If your 3DS travels, each day Fire Emblem should collect a few random Risen battles and one parley party (they look like a StreetPass party, but don't actually come from a Street Pass encounter). Fight all of these guys.
You can also repeatedly summon SpotPass parley parties (free DLC - Wireless > Bonus Box > Bonus Teams) and fight them. There is a wide variety of teams with differing levels, so you should be able to find a team a just the right level to grow your character.
Within each fight, try to weaken enemies with other characters and finish them with Sumia for the most experience. Also, try to position Sumia so that she has a good chance to team up with other characters when they attack an enemy.
FWIW, I found it worthwhile to grind Sumia very early on, to avoid having to grind her child(ren) through the skills which I wanted Sumia to pass down.