This chapter is easy for the first part, but gets more challenging over the course of it. Here's what you need to do: Send one unit to destroy one pillar, and another to destroy the other pillar. That's the easy part. Now, it gets harder. Over the course of the chapter, Axe Fighters and Archers will attack Cordelia. To make matters worse, Cordelia is both a lance user and a Pegasus Knight, so she's at a disadvantage against axe users, and archers deal bonus damage to Pegasus Knights. Send someone to protect her. You will be able to fight Marth after the pillars are destroyed. For Marth's boss battle, use either Chrom, or your main lord.