This battle was pretty short due mostly to a slow-moving, large boss. I'm not sure if he has a number of phases because he died so quickly but here are a couple tips:
- use the Vile Vines Ring that can be found early in the Mire of Stench level (just under where you enter). This ring gives a boost to poison damage, which is the key way the Stench Most Foul will hurt you
- SMF appears to be weak to fire damage, which seems reasonable given his "plant-like" nature
- due to his slow movement, you should be able to deal damage, roll under him when he changes direction, and attack from the opposite side
It was a pretty quick fight; he does low damage (with your defense) and you can do big damage (with buffs).