Fire Emblem: The Sacred Stones/Chapter 8: It's a Trap!

Only send Eirika, and maybe one other character to the southwest to meet Ephraim, as Ephraim's group is enough to take the west path by themselves. Send the rest of your people to the northeast chest, kill the knights, and steal the treasure. Then meet with Ephraim's team in the northwest, after clearing out the main hall. Steal the other chest and beat the boss.