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.