For Jill, supprorting with Zihark works wonders for her, actually USE her. For Leo, supports with Volug helps. Edward and Nolan support well, and when you get to the chapter where all those Laguz units are, when you are using the Dawn Bridage, and you get some NPCs and eventually the Black Knight, I would suggest unequipping his Alondite and telling the NPCs to halt, that was a major help in leveling the DB, especially Jill, and my first use of Jill, level 20/20/20, has all of her stats maxed but skill and defense, and both of those are like 2 short from the cap, she's a beast, I suggest you use her.