Well with the cards and creatures that you already have in this deck you really wouldn't want to splash the colorless in my opinion. You'd be trying to run a four color deck basically for adding a creature type that this deck does not need. I know it may be redundant to say this but
SIMPLE removal is the key. Having a low drop spell like
grasp of darkness,
bone splinters (i really like this card because of its utility),
visions of brutality (they won't DARE swing with their big eldrazi),
unnatural endurance (for your creatures), playing creatures that have first strike AND death touch, or using control cards to bounce big threats back to the hand for a
witness the end are all viable strategies in your color scheme and would not require too much of a deck overhaul.
Also if you add the creatures with death touch in this deck like
nirkana assassin or malakir bat and gave them
pathway arrows then you could literally eliminate ANY threat just by pinging it to death with your creature. Add
retreat to hagra and you'll see faster gameplay in terms of your opponent losing life and also giving more of your creatures death touch. This way if your kalitas or someone gets double or even triple blocked just play
sure strike while it has death touch and you're solid. How does this all sound so far?