i agree with pulling the slaughtermasters, perhaps in place of rampant growth or harrow. Bring up some of the creatures you have to full playset (thoctar, nacatl, loam lion, druid). Sigil Captain is nice but not something i personally would not run 4 of. And finally, swap ajani out for/ add in garruk wildspeaker. will help with your mana fixing, and the next turn can be ultimated for an overrun.
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i played zombies as one of my first constructed decks. you want to play alot of very fast creatures, usually 1-3 drops. try festering goblin its a fantastic one drop. up to 4 barons, there is never a bad time for them to be topdecked. Add in lots of removal for your attackers, and cards like raise deaf for recursion
i used to run a zombie deck, and i always felt that festering goblin was a great one drop. it threw in a cheap, early blocker that discouraged alot of early attacks
i considered coat of arms, but i feel like conqueror's pledge is the better five drop for this deck. i dont want to slog the deck down with too many slower cards