There is a room in The Depths just before you get the bonfire. Run, from the bonfire, to the room, and kill the first Torch Hollow. Sprint to the very end of the room, without stopping and turn in the hallway and kill the other hollow. After that, you should be able to kill off the slimes off with no problem. They are highly resistant to physical damage, but slow to attack. Spells with an AoE will be the most time efficient. Great combustion, Combustion, Chaos Fireball, and Wrath of the Gods are all good examples.
The Swamp Larvae/Leeches in the swamp at the bottom of Blighttown have a rare drop of 5x Green Titanite. Runs here with the Covetous Gold Serpent Ring will net you huge amounts of this ore. Gather whatever poison resist you have. You should be able to survive the run with just your Estus Flasks as you should get back to the bonfire before the poison drains you . Starting from the swamp bonfire, run across the swamp and to the left where you will see your first leech. From there head left hugging the wall and kill every leech you see. Be sure to go in all the nooks. Each run should net you 5-10 Green Titanite, plus a few Large Titanite Shards if you are wearing the ring.