If you are set on running Pharika's Chosen , may I suggest replacing them with the strictly better Ruthless Ripper that will be coming next expansion?

Well since youll be putting lots of creatures in graveyards Avatar of Woe would be a good fit but it would no longer be standard. Sure would be fun with your friends though.

new black discard despise gets rid of planeswalkers out of hand as well and its a common you might want to consider that in at least sideboard, the red is a good idea but it seems like it would only slow things down I would concentrate more on the draw effect the faster you draw the faster you get land and win game and since you have mono black more nythkos shrine

adding to what I said earlier easy sac ability erebos's emissary and also another way to add to the deck doom wake giant you have enough enchantment creatures in the deck it can be useful and cheap since you are playing mono marshmist titan and since you have sac ability with no mana cost you can add scuttling doom engine and champion of stray souls, keep it mono definatly that way you don't have to worry about if you can get the red mana but if you really want to add other colors add gild and king macar, the gold cursed they give you the other color you need and get rid of creatures as well