In War's Grip is Chapter 6x of Fire Emblem: Shadow Dragon.
To advance to this chapter, the number of alive units must be 15 or less by the end of Chapter 6.
Visit the village to recruit Athena. She's a myrmidon who is slightly faster and more skilled than Navarre, though she isn't as strong. If Navarre has surpassed her in terms of speed and skill, then they are going to probably tie in terms of that. Navarre will surpass Athena in strength, unless you have really unlucky growths in strength for him. You should probably use her, but if you want only a couple swordmasters, then leave her be. Est, Catria, Radd, and Navarre can all surpass her if leveled correctly.
This is a good chapter to level up low-level units. Since the enemies are a bit weaker than the ones from chapters 5 and 6, it should be easier to let weaker units get EXP