I've been playing around with mono black on
arena for a little bit.
Torment of Scarabs is infuriating if they can't remove it.
Torment of Hailfire is a game ender, especially with three
Cabal Stronghold.
I know you're trying to keep it more budget, but
Liliana, Death's Majesty as a two-of really helps, too, since it can be a board clear if ulted, or pull a chupa back from yard for another kill (I use the second option significantly more than the first). Also, you can combine her with
The Eldest Reborn to pull her (or, more likely, your opponent's
Karn) from the yard on the third part. Don't underestimate
The Eldest Reborn. It's actually pretty good.