Okay, so to clarify, you're saying this will be played at FNMs and your local FNM features several of the top tier decks, so some combination of mono-blue devotion, mono-black devotion, boros devotion, U/W control, and esper control?
If I'm reading that right, I'm not entirely sure I see how you're performing well. As it stands the deck is currently far too frayed across a smattering of non-optimal creatures and 1-offs. Remember that consistency is one of the hallmarks of a great deck. Now this doesn't mean you need to just drop to a collection of 4x cards, but you should strive toward something like that and very few, if any, 1-offs.
So, how do you improve this to a better, more competitive deck (remember, I'm assuming you're going to be facing the tier 1 decks)? Well, at current there are a few green strategies that have been demonstrated to work effectively: green devotion, naya hexproof (Frank Lepore JUST did a piece on this Friday Dec 20th), Selesnya Aggro, and I even saw a Selesnya Heroic deck that did alright the other day. But, from the look of the creatures you're using, you want to do something a bit different - which is quite alright, as long as you recognize that you probably will not be as successful against the tier 1 decks as you might want.
Cards I would suggest dropping and finding replacements for:
Centaur's Herald - This really doesn't do much for you. If you want centaurs, you can get them via Call of the Conclave on T2 easily. If you want a 1 drop other than Mystic to do something, Experiment One is a better option (and one that fits your +1/+1 counters idea better anyway).
Vastwood Hydra - If you're looking for a hydra to play, Polukranos, Mistcutter, and Kalonian are all just better options.
Voice of Resurgence - doesn't really fit your plan AND can be trade bait for a LOT of pieces you might want to finish off a deck.
Centaur Healer - good SB card, normally not in the maindeck
Renegade Krasis - a decent card for EDH and the like, but it's unlikely to evolve more than once in a constructed setting (unfortunately)
Wayfaring Temple - also a decent card in other formats, but given the lack of token generation here, you don't really need the populate, nor are you likely going to be making him all that big as he's an easy target early on. That being said, if you want to stick with him, you'll need more.
Sylvan Primordial - This is another decent SB card or potentially something you would manage in a devotion deck where mana is in major abundance, but as your deck stands, it'll take forever to cast this guy, likely dropping him after the game is pretty much decided.
Trostani's Summoner - I could see this guy in some sort of a reanimator build going forward, but you never really want to be hard casting a 7-drop that doesn't just end the game.
Druid's Deliverance - I'm normally a big fan of Fog effects, but I don't really see it fitting the strategy all that well...
The artifacts - I'm not entirely sure I can see justification for any of these unless you're looking for some weird combo off Strionic Resonator.
Verdant Haven - great for sealed and draft play, not so much for constructed (unless you're in need of enchantments for some new version of Pillow Fort).
Angelic Accord - Outside of Elixir and Centaur Healer you have zero life gain, meaning this card will almost NEVER trigger. So why use the space?
So that's nearly the whole deck that's sub-optimal (sorry... but you said tier 1 competition).
What cards do you add to make this better? Thankfully there are a lot of good options to choose from given the colors you're in.
+4 Experiment One - evolves, fits the counters, good early game pressure
+4 Kalonian Tusker - pressures the board, triggers evolve on both Exp and Gyre Sage. As an aside, at the two drop you can also use things like Precinct Captain, Call of the Conclave, and Fleecemane Lion
+4 Fleecemane Lion - they're SUPER cheap at the moment (and will go up with Born of the Gods), provide more pressure, more evolve triggers, can be made into indestructible, hexproof beat-sticks (and consequently AMAZING targets for Unflinching Courage)
+ X Witchstalker - a solid option at three, gives you hexproof, a solid body, some hate against control, etc some number would be good, although I don't know that I would go a full 4x... perhaps 3... then again, he might just be for the SB if you want to go a heroic route.
+ 1-2 Kalonian Hydra - I know they're pricey, but this guy is your ace in the hole, so to speak, as one that resolves and gets to swing will frequently win you the game. The full suite of 4 would be ideal, but let's not go overboard here.
I don't want to flood you out on the top end, but really this is where Green shines - Polukranos and Arbor Colossus are both solid options to consider as well. If you're into counters, white really shines with Heroic guys - think Favored Hoplite and especially Phalanx Leader.
Flesh out the other creatures I didn't mention dropping, Gyre Sage, Scavenging Ooze, Crowned Ceratok. You'll definitely want a second copy of Nylea, God of the Hunt.
What else do we add? Well, you need two things out of the rest of your cards - the ability to put some more +1/+1 counters for creatures that wouldn't get them on their own out there AND some protection for your major win cons.
+X Common Bond - if you go with heroic guys, this becomes a HUGE pump spell, but otherwise it can be a combat trick, it can add counters to guys who otherwise wouldn't get them, etc etc. In a worst case scenario, you can use it during the opponent's end-step to force grow your Gyre Sage into producing two more mana the next turn (to drop a hydra for instance).
+X Ranger's Guile/Gods Willing - given the integral role Kalonian Hydra is going to play here, it's kind of a big deal to make sure he lives. Either card will provide some protection, although Willing gives you more options so it's probably the better card to use here
+2 Bow of Nylea - makes even the smallest mana dork into a deathtouch threat AND can put counters on your guys. Probably not the best card in the world, but it's worth a shot.
+X Some sort of removal option - could be Selesnya Charm for BIG opposing threats, Celestial Flare, Renounce the Guilds (although this probably isn't ideal given the devotion decks running around), Last Breath, Pit Fight, Pacifism... something. The point is that you WILL need to interact with the opponent's side of the board given the speed of your plan.
Other cards to consider in some quantity:
Forced Adaptation - not an ideal option, but hey, it's cheap, counters are added... toss it on a mana dork and force them to use a removal spell on it... sure you get two-for-one'd, but that's one fewer removal spell to worry about...
Nylea's Presence - if you're hard up for color fixing this is a better option than Verdant Haven as it replaces itself in your hand via the cantrip.
Garruk, Caller of Beasts - he'd be a HUGE boost, but I recognize he's probably not a realistic option
Mending Touch - regeneration can be huge quite often
Spear of Heliod - not really what you're doing as they're not counters, but anthems are typically pretty good
For the land, adjust once you have the rest of the build pieced together, but you probably will want to splurge for at least 2x Temple Garden. You're really at a disadvantage until Born of the Gods brings the Selesnya Temple.
In the sideboard you can easily add specific hate for various cards, Pithing Needle, Sylvan Primordial, Bramblecrush, Riot Control, Nylea's Disciple, etc.
I don't know that the numbers I gave you above work out to a complete deck (they're probably off by a few cards), but the point I'm making is that you REALLY have to focus on a single strategy and run with it. The deck as currently constructed is just too fragmented to really be that effective. If you want to go a counters route, there's probably a fun build in there that might catch some opponent's off guard, and could even be king of an FNM scene (although unlikely against Tier 1 competition). But, having said all of that, you said you've been quite successful with the deck thus far, so I would imagine the tweaks I've suggested above will push you leaps and bounds in the right direction.
I hope some of this can be helpful. I know it looks expensive, but I'd wager that if you're crafty, you can make 90% of the changes I've suggested for $40ish (which also happens to be the trade value of Voice...).
Good luck!
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