NiGHTS... you really need a little more play experience with gemini. If Gemini are so slow, how is it that Il Blud is run in most competitive zombie decks? Anyway, this deck is incredibly fast: Gigaplant has 2400 Atk and pulls plant/insect types from the deck in less time than it takes to say "ZOMIGOSH A GIANT BUG", especially coupled with Lonefire Blossom. If you can get Gigaplant to turn two, the deck basically has it made. Not to mention, Black Horn of Heaven is basically "Black Horn of Anti-Dragon" since Dark Armed Dragon is special summoned. And as for Gravekeepers, well, if they can outspeed this thing more power to them, but I've never seen a gravekeeper's build actually get five or six monsters with more than 2400 Atk on the field at once before a good OTK deck.
Having said that, Black Horn of Heaven is really better as tech since not EVERY deck runs Dark Armed Dragon. Sakuretsu Armor will really do you no good, since Dark Armed and most other fast decks that dominate this format are either not interested in attacking you or will just blow it off the field before you can use it. Instead, let's try this option, based on a decklist at Metagame.Com by Jerome McHale.
-2 Brain Crusher
-2 Armored Bug
-3 Fairy King Truesdale
-1 Chainsaw insect (Even for 2400 damage, who wants to give their opponent even more CA in an already blazing-fast format?)
-1 Grasschopper
The monster lineup needs a little more consistency- not to mention that Fairy King and Brain Crusher need more tributes than your deck can offer. Besides, you shouldn't be running more gemini than you need to because-lets face it, folks: they're slower on their own.
Now to give the deck some consistency, we're going to add some water-attribute plants and give it a hand control theme (I'm totally ripping this part of the deck off from Jerome here, by the way.)
+3 Homunculus the Alchemic Being
+3 Mother Grizzly
+3 Lekunga
Next, on to the spells. Forest=fail, I'm sorry, NiGHTS is right on this one. Instead, let's try...
-1 Forest
-1 Scapegoat (we're interested in summoning our own monsters, not blocking from attacks, and we aren't running Dogma so they're doing us no good as tribute fodder).
-1 Swords of Revealing Light (Dark Armed is going to blow this away in a heartbeat)
-1 Pot of Avarice (So let me get this straight: you're putting all those plants that Gigaplant wants to summon back into the DECK?)
-1 Lightning Vortex (unnecessary if this works out as planned).
-Book of Moon (sidedecked)
And the traps... let's cut some junk that the metagame can't handle.
-3 Sakuretsu Armor
-2 Negate Attack (Whaddayou think this is, Counter Fairy?)
-2 Magic Cylinder (which last I checked, was still at one.)
-1 Draining Shield
-1 Ultimate Offering
-1 Black Horn of Heaven (sidedecked)
Spell lineup change, anyone?
+3 Creature Swap
+3 Card of Safe Return
And here are a few more cohesive traps:
+3 Spiritual Water Art-Aoi
+3 Solemn Judgement
+1 Torrential Tribute
+1 Mirror Force
+2 Waboku (Dark Armed officially hates this card and its chainability)
Now, this was pretty much quoted (with the exception of Waboku in place of Raigeki Break) wholesale from the Jerome McHale deck. There is an OTK version of this deck focusing on a Mass Driver loop- summon Superalloy Beast Raptinus with dragon's mirror before summoning Gigaplant and reviving another one (or summoning it from the deck with Lonefire Blossom) and creating an infinite loop in which you tribute one Gigaplant for Mass Driver before summoning it back to the field with the other one. Creative and painful, I'd say.
As for this deck... well, I'd say that it needs more work than I gave to it (I mean, come on, the fixes weren't even my own ideas mostly). So anyone else feel free to add to what I've put here.