I think you're talking about the Rusted Iron Ring?
This ring is pretty useful as it eliminates the movement penalty from moving around in water, or the swamp in Blighttown.
To get it, you must unlock that door using the Undead Asylum F2 West Key. This key is found in the Firelink shrine on a roof near the area you must navigate to return to the Undead Asylum.
This video shows you how to get back to the Asylum. You can also use this written guide on how to return to the area (courtesy of the Dark Souls Wiki):
"To do so, you must have access to the elevator that goes down to Firelink Shrine from the Undead Parish.
Get on the elevator from Firelink Shrine and, as the elevator starts moving up, you must move forward and drop on top of the area where you exit/enter the elevator at the bottom. Follow the path to the right, on top of a little green patch, then look down and you will see a broken column. Roll towards that broken column and land on it, then go across the beam. From there climb up the stairs all the way to the top where you will find the Snuggly the Crow's nest. Step in to it and you will get a prompt to 'Curl Into a Ball'. If you crawl into a ball and wait about 30 seconds, the Crow will pick you up and bring you back to the Undead Asylum."
Since you can freely return to the Asylum whenever you want to through this method, I do not think any item is "missable".
There's two more places I can think of. In order of progression:
Painted World of Ariamis
This is the place where I did my farming after Dark Root Garden wasn't cutting it anymore. First, you will need to:
Find the Peculiar Doll in the Undead Asylum on your second visit. For instructions on how to return to Undead Asylum, check this video. The doll can be found in the same cell where you started the game.
Reach Anor Londo and locate the big room where the Painted Guards are. You will find a huge painting on the wall. Descend to the floor level and touch the painting to be transported into the Painted World of Ariamis.
Progress through the level until you reach a sort of town square, where you will find a slew of Phalanx mobs. These are the creatures you will be farming.
Across this courtyard, you will find a door that opens up and leads you to the starting bonfire.
You will need an AOE damage spell for this method. Poison Mist is a good choice for low levels, since Phalanx are weak to poison. They move slowly, so you can run up to them and cast Poison Mist one or two times while moving in circles around them. Just wait for a few seconds and they will die. With the Covetous Silver Serpent Ring equipped, this method will net you around 9000 souls in a few seconds. Just run back to the bonfire and repeat.
If you are a caster class, you will be able to speed up the farming by using strong AOE spells. For instance, I play a faith build character. My Wrath of the Gods spell managed to kill all Phalanx with a single cast, netting me 9000 souls every 20 - 30 seconds. I spent more time running up to the town square than killing them. The Homeward miracle will speed up this process considerably.
Now this method requires you to side track from the main story progress. Another method you can try, which doesn't require you to do any side-quest is farming Dark Anor Londo.
Dark Anor Londo:
After beating the Ornstein and Smough encounter, you will meet Gwyneavere. If you hurt her she will disappear and Anor Londo will become Dark. Warning: Making Gwyneavere disappear means you won't be able to join the Princess' Guard covenant for the rest of your current play-through.
In Dark Anor Londo, only Painted Guards and Silver Knights will remain, the rest will be gone, including the giants and archer right outside Ornstein and Smough's fog wall. Instead, you will find two NPCs here. You can kill them for around 6000 souls with the Covetous Silver Serpent ring equipped. The trick is, they will only respawn if you leave Anor Londo and return. However, you can use Homeward Miracle to return to the previous bonfire where you rested at. The NPCs will also reappear.
Best Answer
Well, depending what you mean by "head on", it is possible to melee the boar and perform back stabs. If you aren't heavily encumbered, it's pretty easy to maneuver behind him and it should only take 2-3 back stabs.
Another option is to throw an alluring skull on the bonfire. He'll damage himself by running into the flames.