Go to the tunnel by firelink and buy purging stones, purple moss, and blooming moss.
When you're in the Depths, don't stand in the Basilisk smoke. Ever. If you muck it up, purging stone. Dilemma solved. Honestly though, I don't think you actually have to fight a single one. Only if you slip and fall in a hole.
If you get poisoned, it honestly doesn't really matter, fight the enemies or get out of the swamp, get to safety when you can, and then use a moss.
If you get Toxified, get away from the arrows immediately and use a blooming moss. Rock a spider shield to 100% block them.
All problems solved.
New Londo is a hell hole. I hate that place. At least it runs at the right framerate though, lol.