There are a couple of card choices that I think could be improved. I'll try to pick them all out. I also have my own Damia deck and I'll try and suggest some stuff.
Also let us know if you want your deck more focused on multiplayer or 1v1.
Boomerang is okay. but
Capsize is almost always better. Since you have black why not
RecoilMana Leak is weak late game. replace it with another hard counterspell. For Damia I really like
Forbid.
I would replace Rampant Growth with
Kodama's Reach more land is more better.
I think sign in blood is a bad idea. its double black and you pay two life. You have blue card draw available, find something there. I would run
Diviniation over sign in blood. On that note, I also think you could replace Vision Skeins.
Underworld Dreams seems weak when an opponent starts at 40 life.
Having had Wild Mongrel in my own original list, I have to say I have never been happy when I drew him. You might think he is great cause you can make him huge with Damia out, but he is pretty boring without her. (Even with her he doesn't do much.)
I'd like to 2nd that Nirkana Cutthroat is just bad. There are many better options. I wouldn't even play Vampire Nighthawk. He is too small and too weak.
Jace's Archivist seems like you are just gonna end up helping your opponent get cards. Explain him, cause I don't get it.
So here are a few cards in my list that really shine for me.
Any of the titans in M11 or 12.
Dread Cacodemon Pestilence Demon Reiver Demon are all helpful to control the board.
Sheoldred, Whispering One and
Garruk's Horde are both really good and pre release cards so they shouldn't be too hard to find and trade for.
Praetor's GraspMind Shatter and/or
Mind TwistUrborg Elf Best Damia elf there is!
Birthing Pod is really good. I do think has gone up in price though.
Acidic Slime can kill just about anything. This is a staple green card in my opinion.
Solemn SimulacrumCloudthresherOverbeing of Myth With Damia out you skip that extra draw but the Overbeing of Myth is gonna be a big beater and its card advantage while Damia isn't out.
Trygon PredatorPutrefySoul ManipulationMaelstrom PulseShriekmawYavimaya Elder<---another green staple.
Wrexial, the Risen Deep Way fun EDH card. He also works with Windfall and Jace's archivist if you want to keep those guys in. If you do, consider adding
ProsperityBrutalizer Exarch is great. lots of versatility.
Deadwood TreefolkAether Mutation This is great when you bounce a real big creature of your opponents. However is can also work well with creatures like
Mulldrifter Shriekmaw and
Acidic SlimeThis last set of cards are pricier, but others might be willing to drop some money or trade or just happen to have these.
Hornet Queen is in one of the commander decks.
Spiritmonger this guy is AWESOME! 6/6 for 5 that regens and can change colors!
Pernicious DeedCadaverous Bloom This card isn't a lot of money but it might be harder to find.
Mystic Snake I love this thing. leave a mana open and birthing pod it into play, they'll never see it coming.