This has been my strategy:
On first getting to the tactics screen (where you pick units and items), I check the names of the units (knights and non-knights), then check my inventory to see if I have any key items for recruiting. If it's a non-knight, they'll give you a weapon (that's all I've got so far). Then I check the placements of chests/boxes/other things to break in relation to the units. Let's say there's a rock next to one. I'll stick a warrior there (or anything else that can hit it) to try to get the item from it. If something is out of range, I try to find a unit that can hit it from a distance, or pick a unit that can move there. Duelists and L. Knights I know so far can move squares (apparently some items can do the same, but I'm yet to see any). Then after a turn, I can try to find something than can hit it from the new spot. After that, it's the usual picking of weapons that match the units and their levels. I don't really concern myself with matching elements much. If I can, cool, but otherwise I don't really bother (unless it's nearing the end of the turns).
First things first, I use my key items to get those out of the way. Sometimes I have so many, that I'm left to only be able to build MP. That's not such a bad thing, really. In battle, I of course do a regular attack (just hold down on the unit and pull a direction) and try to hit as many enemies as possible to build up MP. After that I pick weapons that I hope will do a lovely job of smearing enemies across the board. At the same time, I try to hit as many of those chests/boxes as possible for key items. If I need to move units, I'll do that, too.
After that, it's just a matter of if you want to get all the items on the board (I very rarely can even get them all), or if you just want to kill everything as fast as possible. Just keep in mind, the more you recruit, the more classes you'll have to use all your weapons. And the more enemies you kill, the more weapons you'll earn.
Just remember kids: distribute that EXP between chapters!
