Try to keep every character alive, you may never know who you need to recruit another unit.
Level up units to level 20 before promoting them, this way you can get the best out them as they will gain more stats.
Except in the case of clerics/troubadours you may want to promote them at level 10.
Try to level up a lot of characters, especially the lords as you will need them the most in the last few chapters, you need many strong characters to beat the last few chapters not just one or two.
Weapon triangle + Terrian
Always use terrian to your advantage this way it will be harder for you to get hit/take less damage.
And remember to use the weapon triangle it will help you gain more attack, hit and critical, and increase your tactitions rank