Thankfully, at this low level, Volcarona is a bit short on powerful attacks, mostly using Leech Life and String Shot- not a winning combination by anyone's estimation.  It can also apply Gust with a bit of vigor, but loses the type advantage on that.  With a bit of endurance and luck (and probably a fair number of potions) you can capture it to add to your team or collection.

Head South back out of the room, then resume traveling East.  You'll eventually reach a room with a sandpile, where you can pick up an Ultra Ball (possibly useful for Volcarona).  This is the end of the Relic Castle, though, so head West out of it and pick up where you left off spelunking.  Travel South as far as you can, keeping East, and you'll come to some stairs up to another tunnel.  Enter and follow the path and you'll come to another Strength rock.  Shove it into the hole to open up a path North, avoiding the ledges that will return you to the Castelia Sewers area of the tunnel.

This new path North turns West after a while and heads through a passage into a small cave you passed by earlier.  Here you can pick up an Ultra Ball, a Rare Candy, and a second Rocky Helmet.  Also present, though hidden on the floor, is a Rock Gem.

Rare Candy are an immense boon to your game when used right, so this is a precious find.  When used on a pokemon, a Rare Candy will give it exactly enough Exp. points to advance to the next level.  While this can be wasted by using it on a pokemon that is low level or about to level up anyways, there are many other situations where it's a hugely valuable item.  For one, since Rare Candy don't care how many experience are needed for a level, they are a quick way to level up a pokemon that's so high already that it's hard to advance with it at all.  If you're just barely stymied by the power of an opponent or legendary pokemon at a point in the game's story, you can reload your save (or not) and apply a Rare Candy to make up the difference.  If you're out in the field and a valuable team member is knocked out at just the wrong time, you can use a Rare Candy instead of a Revive- the pokemon will regain consciousness with as many HP as it gained from leveling up, so it will need more healing items, but it is a useful alternative and if you're lucky, the pokemon will learn a new move that may prevent later knockouts.  Finally, post-endgame when you're training pokemon to advance their Effort Values, Rare Candy will let you boost them to a level where they can handle whatever you want to train them with, or can be used after the Effort Values are all in to push them towards level 100 without spending hours grinding Audino in the middle of nowhere.

The Rock Gem is less valuable, but still very interesting.  The Gems (there is one kind for each element, named appropriately) are hold items for pokemon that are used only once- the first time a pokemon uses a move of the type noted by the Gem's name, that move's power is increased by half (x150%, essentially) and the item is consumed.  This is particularly interesting when combined with moves like Acrobatics (which powers up when the user isn't holding an item) or pokemon like Unburden Drifblim (which gets much faster if it is holding an item and the item is used, dropped, thrown, or otherwise goes away).