Weapon Triangle
Keep the weapon triangle in mind. Swords have the advantage against axes, lances against swords and axes against lances. This weapon triangle is your most powerful skill besides unit placement in the game. Some weapons, however, reverse the triangle, such as the Lancereaver sword, which defeats lances rather than the original axe advantage. An axe such as the Axereaver can perform the same action like defeating swords rather than lances.
Unit Placement
Pay attention to where you place your units. Depending on the class they are, their stats will be varied a great amount. Put ranged attackers in the back, as their defense tends to be lower than melee fighters.
Elevation and Rear/Side Attacks
If you can, take the higher ground; it gives a chance to attack and sometimes leaves the attacked unit defenseless to counter. The side and rear of a unit gives a greater chance to hit and possibly score a critical hit, so place your units to the side of and behind your enemies.
Risk Taking/Retries
"Radiant Dawn" contains a "battle save" feature that allows you to save your game in the middle of mission. It can also be used as a place to just save in case you're willing to take a risk against the odds. Taking risks can benefit you very much, as well as possibly making you lose a character. "Fire Emblem" risks are very common, and the battle save feature can help you with that. If you are not sure about a fight about to happen, battle save; if the fight doesn't turn out the way you hoped, reload your battle save.