Strategies for all Difficulties
This map has three choke points, being the west (where Chrom starts off), east (where your second group of units are), and center (where Marth is standing).
Gaius is in the west and since he moves up toward you, it is best to leave Chrom there to recruit this thief character earlier in the level. The south choke point is best defended by a speed/resistance pair, such as Sumia/Frederick. Placing units to block incoming enemies is the best defensive strategy to do. The boss will eventually move himself up and enter likely through the west side.
It’s okay if Marth kicks the bucket, but be wary about letting enemies slipping past and reaching Emmeryn. Doing so puts you in danger of a Game Over.