This deck has a pretty straight forward game plan. Get some creatures with counters in play, get Gilder Bairn into play with a way to tap it, then double counters as many times as possible per turn.
Threats
This blue deck doesn't have much draw power, instead relying on the idea that most of the creatures in the deck can be turned into massive, deadly behemoths if given enough time and attention. This means that as soon as your opponents run out of creature removal, players usually start dying. However, there are a few all stars that are especially good, thanks to their speed or high impact. Cloudfin Raptor and Battering Krasis are excellent because they're cheap and will quickly gain one or two counters without investing any additional mana into them. This combined with their evasion/trample makes them some of the fastest game ending threats in the deck. Other powerhouses include Ivy Elemental and Cytospawn Shambler that are amazing late-game drops since they will come down pretty big so that you don't have to devote many Gilder Bairn activations to them before they're able to one-shot opponents. Another great threat is Jade Guardian, since hexproof means you can take your time building it up without worrying about removal.
As an alternate way to build threats, there are also some token producing thallids that will allow you to create massive token swarms. This is to try to prevent single effective blockers like Dream Fighter from stopping our advances for too long. These are also nice because even if they get targeted with removal, you can unload all their counters at instant speed to create some saprolings for chump blocking or chip damage.
Don't put all your eggs in one Basket.
One of the main tenants of the deck is to try to develop multiple threats at once so that a single removal spell doesn't completely gut your board state. To help with that, we have many ways to either put counters on multiple creatures at once, like Thrummingbird or Fuel for the Cause, and cards that help move counters around, like graft creatures, Spikes, and Scrounging Bandar. We also have several triggered abilities like Ivy Lane Denizen and Saddleback Lagac that help to get the first counter on more creatures that might otherwise be around just for utility.
Another idea that plays into not over committing to a single threat is that you can activate Gilder Bairn at instant speed during your opponents' turns, in response to removal, attacks, or end steps. This helps create surprises, makes it harder for your opponents to know where to aim removal, and also makes you look less threatening than you actually are. However, you might want to forgo this in favor of doubling counters before attacking if you're trying to race to kill a combo deck or knock out an aggro deck that has you in its sights.
Untappers
We have 14 repeatable ways to tap Gilder Bairn without even having to attack (although that's always available as a backup option). These range from mana production like Loam Dryad, to interaction like Viridian Longbow and Rhystic Deluge. I think these untappers are what really makes the deck great, since you're getting a lot of value out of what should be a negative cost. The fact that several of these also are interaction also makes the deck much more effective at killing or controlling enemy threats than the average green stompy list.
The real goal with these untappers, though, is to activate Guilder Bairn multiple times in a turn, which is usually doable by around turn 6 or 7. At this point, the game can rapidly shift in our favor as we create unstoppably large threats. Ideally, though, you still want to have backup plans brewing, getting your next threat ready in case your biggest beater bites the big one.
Interaction
As I just said, we have a pretty decent suite of ways to mess with our opponents plans. We have removal for small creatures in Viridian Longbow and ping auras like Hermetic Study. These can do a surprising amount of work since they will pick off a 2/2 each turn once we are getting two activations at a time. We also have a lot of tapdown effects, like Narcolepsy, Leonin Bola, and Rhystic Deluge. We even have repeatable removal for artifacts and enchantments in Wickerbough Elder. Just keep one counter on him and double it right before you remove a counter to destroy something.
Alternate Win Conditions and Tricks
There are two non-combat win conditions in the deck to get around fog locks. The first and simpler of the two is Thorn Thallid. Just pile spore counters onto him and he will quickly be like a loaded Aetherflux Reservoir, just waiting to kill any opponent that crosses you at instant speed.
The second is actually an infinite combo. By getting at least 3 counters on Pentad Prism, you can double the counters on it and use the resulting three counters to pay for the next Gilder Bairn activation, continuing the cycle. If you start with 4 counters on the prism, this also gives you infinite mana. Even if you don't have the infinite mana, infinite untaps with Gilder Bairn turns any ping aura or equipment into infinite damage to every opponent.
Future Improvements
Honestly, this deck worked so well from its inception that I haven't put as much time as I should have into refining it. One area that I want to improve in the future are refining the suite of small creatures that will help develop your first threat sooner. Another is that I want to incorporate more instant speed spells so that you can hold mana for them and if you don't need them, then the mana gets dumped into a Gilder Bairn activation during your last opponent's end step.
