Hey you!
I copied your deck and did a little bit of touching up to it on my profile. ;)
I took out a lot of the singles and riff-raff and added cards that I think would really help your cause. Here's what seems to be the foundations or win-conditions of your deck:
1) Mill them down with Deathcult Rogue/Paranoid Delusions
2) 'Corrupt' them with Grey Merchant & Corrupt
3) Create board control with critter destruction (Doom Blade, Ultimate Price) & critter control (Nightveil Specter & Ashiok)


What I've added to the deck is this:
1) A little more mana. 21 is just not going to cut it, so bumping it to 23 sounds a lot more stable.


2) Cutting a lot of double-blue spells & one-copy blue spells will help the deck continue to flow, allowing you to cast a card nearly every turn.


3) You're absolutely right that you need smaller-costing cards, that's why additions like Devour Flesh are needed for destruction rather than the gargantuanly expensive Sip of Hemlock.


4) Critter destruction is much more explosive and devastating when it is done at 'instant' speed. I eliminated sorceries and enchantments that slowed down the pain & added more copies of Corrupt & instant kills.


5) This deck now has SO MANY options with the additions of Liliana, Sanguine Bond, & Rescue from the Underworld. Liliana brings you constant land in your hand with help fuel Corrupt, her middle effect can act as either critter killing OR a massive swing of damage via Deathcult Rogue or Nightveil Spectre. Sanguine Bond is just explosive for your 'Corrupt' spells & even the Elixir! Imagine dealing twice the damage with Corrupt! It's a massive swing of life totals or a potential kill card! Rescue from the Underworld is a much faster and potent spell than March of the Returned.


I hope this helps somewhat ;) Again, the deck is in my profile and you can just copy it over to yours.