After emptying out the small cave of valuables, follow the tunnel back the other way.  Again, avoid the ledges unless you really like the Castelia Sewers, and make your way back to the hole you used to enter this passage in the first place.  Go down the stairs and follow the tunnel West and then North to encounter another backpacker who's apparently obsessed with echoes.

Battle:  Backpacker Annie (768 Poke)
Opponent: Watchog Lv 32

On the rock past Annie is a Yellow Shard, so grab that and continue North.  Follow the tunnel as you backtrack, until you get a chance to turn to the West.  Take that opportunity as soon as you can and you'll come to another psychic and another rock.

Battle: Psychic Tully (1056 poke)
Opponent: Swoobat Lv 33

The rock is holding up a Green Shard, which you should snag on your way back to the West.  Keep going West (and, when you need to, North) until you see the water again.  Head South along the East edge of it, between the pool and the large rock.  You'll find a dead end leading back to the East, which contains a Max Repel.  This is the longest-lasting of the Repel items, which will, for a set number of steps after use, keep you from encountering wild pokemon of a level lower than that of the pokemon in the first slot of your team.  Useful when you're traipsing back through old areas and don't want to get bogged down in battles, but much less powerful traveling through the story, as it keeps you from the wild battles that should make up a significant portion of your team Exp points.

Head back West to pick up a Blue Shard, the last item in this neck of the cave.  Go back around the pool to the North, and on up the stairs.  Keep returning Westward, shoving the boulder into the hole and taking the Southern path to encounter yet another backpacker.

Battle: Backpacker Eileen (768 Poke)
Opponent: Sandslash Lv 32

With Eileen flattened, you've done everything you can do in the passage for now, so step back outside and take a rest at the Pokemon Center before returning to the tournament.