After getting some unexpected help from your enemy, you seek the Rainbow Sage's aid.
Rout the Enemy
In this battle, you will send your units up the stairs to make their way to the top. You will need to split up your team, fighting the enemies that block the stairs. Then send your units one-by-one up the stairs.
Be careful not to accidentally tell your units to wait on the stairs. If you do, the stairs will be blocked until the next player phase.
There are a couple of chests, one on each side of the building. An enemy in each of those rooms will drop a chest key, so you will still be able to open the chests even if you don't have a unit that knows Locktouch.