Ensnaring bridge is great and it's a four of in this deck without a doubt. You NEED that to be on the board and if it gets removed you need another one to hit the board.
The problem with the way you've built this is that there's no removal for a threat that does make it to the board. You don't have control over the next card they draw and they can play whatever they have at the start of their next turn. If they resolve a threat it's going to go un-checked for the rest of the game.
Check out my mono-black discard deck. I've done pretty well with it.