Got this last week and have been loving the hell out of it. Any tips on how to get low level grunts up to speed with everyone? At the point I got donnell and maribelle they get one shot by everything.
The free Marth DLC (Champions of Yore 1) that you can download is excellent for this. There's also something called EXPonential Growth to level off of, but that's not viable until a bit later, and it also costs money. The enemies all run from you in that one but they'll still counter you during the fight. Chrom is my second highest char and a full hp Chrom with support went in and it told me he was likely to die, heh.
But the Champions of Yore 1-3 are excellent for leveling lowbies. Put them with a support char that you intend for them to marry so you can build up their units too. Once they get a few levels, they can survive in Champions of Yore 3 (Champions of Yore 2 is very fast; if you take just one unit in you're probably fine but all of them require Chrom goes in and those lowbie units would just throw themselves at him and die instantly.)
Also, Maribelle and Lissa as healers are easily leveled with rescue staffs. When I had to level them quick I went into a Champions battle and just had them stay behind, using the rescue staffs on each other. They give you ridiculous amounts of exp per use. They are expensive to buy though (The rescue staff) but it worked for me and I hadn't even done the golden gaffe, which is like EXP growth except when you kill the running enemies they give several k gold every kill. I found this a better strategy than healing staffs, as you don't need anyone to be damaged to use it, and if you have them running around healing people they're generally in range to get pummeled. It also means if you're trying to level up non promoted chars, you can leave them in a safe area where an enemy won't challenge them. That all gets a bit easier if you promote Maribelle and Lissa to the classes which can use tomes. (Valkyrie and Sage respectively.)
When leveling donnel I put him with Kellem for awhile - Kellem's def stats made it so Donnel took a lot less damage, then I had a healer just follow him around healing.
Anyways, the key to all of that is the Champions of Yore Xenologue, which is free to download until March 6th, so if you haven't done it, go get it.