You'll have to contend with some particularly poisonous enemy types in the Black Gulch section of Dark Souls 2. With that said, there are a few handy items you can use to both slow down this damage, and heal yourself through it. Our guide to this section of the game will get you through the safety of the shadows, and into the fight against the filthy Rotten.
The Black Gulch guide
As soon as you've entered the Black Gulch, make your way over to the cave on the left-hand side. Once inside, light up the bonfire and then prepare to tackle a deadly pack of poisonous enemies just up ahead.
Make sure you smash up any of the poisonous statues you see as you head down the path, so they can't fire their venom at you. Keep in mind that they'll even try to spit at you while you're killing them, so make use of any Common Fruit you have on hand to slow the poison down. A dab of Poison Moss (like the stuff you picked up in the last section) will heal some of that poison damage after the fight.
Eventually you'll come to a set of black holes. Approach them extremely carefully, and then chuck a Firebomb into the hole in the centre, then the hole on the right. This will set fire to them both, and take a chunk of health off the enemies hiding within. They'll leap out pretty soon after the fire lands, so get ready to fight them if the Firebombs didn't finish them off.
"If you're out of bombs, inch your way close to each hole in turn, then dart back quickly to lure the creature out into the open."
Now make your way over to the left of this first set of dark holes to find a cave. Kill the poison-spitting statue by the entrance, then creep inside and open up the coffer to get a Great Magic Weapon and a Shotel. As you exit the cave, there's an enemy hiding inside the first hole on your left - kill it. If you don't have solid ranged weaponry to hand, make your way forward very, very cautiously, as there are more statues littered around the upper ledge area to your left.
Once you've made it past this fresh crop of statues, approach the cave on your left. Inside, you'll have to fight a worm creature that's just a little different from those that lurked in the holes earlier on. If you get too close to this beast, it will lunge through the cave entrance. Quickly jump back out of range of this attack, then dart straight back in and whack the beast when it fails to connect with you. Keep repeating this cat-and-mouse process until the creature falls to your sword.
Next, smash up the pottery that's by the cave and you'll be able to loot Scraps of Life, a Torch and a Soul of a Nameless Soldier. Just be ready to attack another creature that comes from the darkened hole on the left-hand wall.
After you've passed this threat, you don't have to worry about the hole in the floor just up ahead, but you do need to think about the trio of holes behind it. The two on the right contain creatures which need to be Firebombed. If you're out of bombs, inch your way close to each hole in turn, then dart back quickly to lure the creature out into the open. When you come to the next row of black holes, be ready to fight the creature hiding in the central hole.
The next set of pottery you come across is well worth smashing up, as you'll loot a Radiant Lifegem for doing so. Grab it, kill all of the nearby enemies, and then enter the cave that's over to your left. Make sure you smash up all of the statues in here - including the one just behind the chest - then take the Divine Blessing from the chest.
Leave the cave now and kill the creature that you discover in the dark hole just over on the other side of this area. That's the last of these sneaky blighters you need to worry about, but before going through the fog door, face away from it and follow the left wall right the way into the far corner of this open patch of Black Gulch, beyond the black holes. If you look carefully over the edge, you will see a little path leading up the side of the cavern wall. Follow it and it leads to a bonfire that proves very handy if you die facing the boss.
With that bonfire lit, drop back down into the main Black Gulch area, dodge past any respawned enemies and head on through the nearby mist to take on the might of the Rotten.