Why not try Triumph of the Horde since it gives all creatures +1/+1 trample and infect until end of turn
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I really like this deck however like you say it fails against leyline of sanctity it also doesnt do well against the melira + phyrexian unlife combo so i why suggest splashing white for enchantment kill
I would suggest adding more equipment to the deck otherwise Puresreel Paladin wont be doing very much
I really like the idea of using phyrexian mana cards to cut your life quiker however i still think you need a way to protect Melira so i would suggest either removing the 2 Cathedral Membrane or 2 Norn's Annex and put in 2 Darksteel Plate other than that I really like the deck
I sugest adding in more proliferate into the deck try using Tezzeret's Gambit, Steady Progress or what I find works really well is Inexonorable Tide
I would recommend playing either Ascetism or Darksteel Plate to help protect Melira. Why not take a look at my Melira + unlife deck http://www.mtgvault.com/ViewDeck.aspx?DeckID=186178