So is this for kitchen table play (casual) or are you hoping to fare well at FNM with it?
Some more general observations:
- Heroic is really best in the realm of x/W decks as they have CHEAP heroic guys and a bevvy of protection spells that are only 1-2 mana (think Gods Willing, Ajani's Presence, Feat of Resistance). Moreover, those spells that give protection to a color end up providing a nice way to alpha strike the opponent with your giant heroic guy.
- Hornet Nest is a solid way to stonewall aggro decks, but unless your meta has a ton of them, it really doesn't make much sense in the main. Additionally, it's the perfect card to run with fight effects (see Setessan Tactics) as you can FORCE the opponent to give you hornet tokens.
- I like the idea of Kalonian Twingrove but I don't know that I'd run 4x. The same can be said for Sandsteppe Mastodon. Admittedly you have SOME ramp, but not enough to have 8x creatures with the CMC that high.
- If you're mono-colored with Theros legal, you should be running Nykthos, Shrine to Nyx in some quantity.
- Polukranos, World Eater really is THAT good and should be in here as a 3-4x. If you don't have them and don't want to drop the cash Temur Sabertooth offers a slightly weaker budget alternative that opens the door for some other lines of play. Poluk is better, but the cat could work in a pinch.
- Naturalize and Desecration Plague are both sideboard cards. If you aren't staring down enchantments they end up as dead cards.
- You really need some form of card advantage as the biggest problem mono-green runs into is when your hand is empty and you have nothing to do with your excess mana. Shamanic Revelation is a decent budget option here. Just be sure to cast it BEFORE the opponent wipes the board. Even if it only nets you 2-4 cards, it has replenished your hand enough to keep the game going.
- I think the counters strategy you're looking for here is a bit more gimmicky and cute than effective. Hardened Scales invites the 2-for-1 and the other spells really just don't get as much done as you'd probably want. Green's real strength is in having creatures that are just larger than the opponent by default. Why invest 4 in Staunch-Hearted Warrior and another 4 in Dragonscale Boon to produce a 7/7 when you could just as easily pay 6 for Hydra Broodmaster for a 7/7 that gives you added options as the game progresses?
- Obviously Nissa is good in a mono-green build, but she's pricey, so if you have em, run with it. If not, don't stress it too much.
I'd suggest you take a look at the mono-green lists out there that are seeing play/press on the major sites and use those as a template. I understand how budgets work and that not all of us can go drop $150 on singles to keep up with the Joneses, but the pro builds can at least give you a point of departure to start from.
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