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 ephraims group is enough to take the west path by themselves. Send the rest of your people to the north-east chest, kill the knights, and steal the treasure. Then meet with ephraims team in the north-west, after clearing out the main hall. Steal the other chest and beat the boss.