day of judgment + mana leak + jace unsummons
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You have 2 bull cerodons, I might consider adding Conqueror's pledge. Also spot removal like Path to Exile or Condemn is invaluable in edh, you'll be up against all types of terrible threats. Glorious Anthem is also nice. Sword of the Paruns would work very nicely too. I would also consider Sun Titan.
For a poison deck this has remarkably few ways to deal poison (3 specters, 3 crusaders). I would cut out the remember the fallen's at the least to add in 3 higher quality poison creatures. I would also see if porcelain legionnaire could be replace with a good infect creature.
I feel like Oona is too slow, you should really look into mono-blue sanity grinding as a strong milldeck. Also 20 lands is too few when you have 8 5-drops and 4 6-drops. You should probably scale that up to 22-24 at least.
there's nothing else to really proliferate, the idea is that semblance anvil +architects + chalice provide enough ramp reliably to get the big guys out reasonably early and often
Moltensteel dragon = swing for 16 the turn after it comes out, assuming you played it for 4 and 4 life (pay 14 life for +7, tap 5 mana for +5 = 4+7+5= 16) Definitely put this in as a finisher if they last to turn 5.