From the staircase, head south and follow the path. Before you can open the chest you come across, the LancerX nearby will stop you.
This guy is undoubtly fast, and will usually get the first attack off. But he is alone, and will fall quickly to your whole party. Have Nina simply use the FlameRP and the rest should attack. He should fall in about two rounds, netting you 650EP/442GP.
Afterwards, open up the chest for a Herb, and then follow the path north, ignoring the weak walls you see, where you will come across another LancerX.
Like the last one, he is fast, but if you combine the FlameRP with your attacks, he'll go down in about two rounds. You get the same reward of 650EP/442GP.
After your victory, open the chest for another Herb, and continue following the path where you will come across two weak walls. Have Ox knock down the first weak wall (or the one to your left), and enter the once blocked passageway. You should eventually come acorss a chest containing a Life. Head back to the area where you knocked down the wall, and head east. Open the next chest you spot for a MetalSH. If you have an IceSH on everyone equipped (LightSH on Nina), then most likely you can sell it off for extra gold. Continue up the path to encounter another LancerX. This is the last one in the dungeon, so make it a good one!
This is the last guy! Whack him with a FlameRP and a few attacks and he'll be down like the rest! The usual 650EP/442GP is your reward.
Afterwards, continue up the path and break the weak wall with Ox. Head east, and down the path you see. When you encounter another weak wall, have Ox break it, and head up the path north to a staircase. Heal up, and head down the staircase.