Hey Dolf,

This is a standard deck I have been working on and it works pretty good at FNM, but it has problems with fast decks, flying decks and has no sideboard.

Instants and Sorceries

4 Overwhelming Stampede
4 Lightning Bolts
4 Fireball
4 Cultivates
4 Explore

Creatures

4 Llanowar Elves
2 Jorga Treespeaker
4 Rampaging Baloth
4 Engulfing Slagwurm
4 Omnath Locus of mana

Land

16 Forests
4 Mountains 
4 Rootbound Crag

Maybe Board
Steel Helkite
Natrulize (sideboard)

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Ethan,

Thanks for the deck submission and info. When you're able to let me know exactly what is causing you problems like this it makes it so much easier to try and fix it. Looking over the list it's fairly solid and I can definitely tell that you had a specific goal and built towards that. Specifically you're looking to pump out lots of mana and use it to get out some nasty critters quickly and finish the opponent with a Fireball or some trampling goodness thanks to Overwhelming Stampede.

Overall you have a pretty good list here and the idea behind it is solid. On a card for card basis your creatures are likely going to be better than what you'd see from the more aggressive decks you have problems with. The problem here being that they can put a lot of pressure on you very quickly and kill off your little mana producers to hamstring you while bashing in for the win. To counter this what I'm going to recommend is that we make your own deck more aggressive while still utilizing the benefits of having lots of mana.

First thing we want to do here is cut your deck down to 60 cards from 62, you really want to always keep your decks to 60 cards as a rule of thumb simply because it's going to force you to evaluate every card and only keep the best ones. It then also increases the chances of you drawing your best cards because there isn't extra filler. As such I'll be making a couple cuts likely to the higher end of your curve.

To get the more aggressive theme going we're going to want to dip slightly more into red so that you can run Plated Geopede. The geopede has great synergy with this deck as you're going to be playing lots of lands anyway which will pump him up. He's also a two drop with first strike so he can give you the early game presence you need. The other change we can make here is to cut some copies of overwhelming stampede and bring in some copies of Garruk Wildspeaker.

Garruk provides a similar effect to the stampede with his ultimate ability but also provides much more utility. His other abilities will allow you to ramp up more mana or create 3/3 beast tokens which you could use to contend with those aggressive decks. Another card we could put in here would be Obstinate Baloth which comes down on turn 4 (likely sooner here), has a decent size body, and nets you 4 life which can be crucial against fast aggro decks.

The last thing I'll mention to counter aggro decks is of course... Pyroclasm, which will wipe the board of all their weenies. The only problem with this being that it will also hit your own little guys (though if you played a land that turn the geopede will live). This can thus be a sideboard card and you shouldn't need to use it unless playing against some really insane ultra aggro decks in which case yo might want to bring out the tree-speakers and some other high end spells to board these in. If the llanowars can ramp you into even one decent creature or spell they've served their purpose and are worth the sacrifice if you need to pyroclasm.

Now for the other weakness. I was looking through to see what we could do to counter heavy flyer decks and a couple of cards stood out to me that will likely reside in your sideboard until you need them. The first is Plummet which is a pretty straightforward creature kill card for flying creatures and will take down anything your lightning bolts can't. The other card is Gravity Well. This card will pretty much make it so that if your opponent wants to attack you his flyers will be forced to engage your beefy creatures to do so. You may also want to try boarding in the pryroclasms I mentioned above if they're smaller flyers.

Let's take a minute here to discuss Omnath. Because we're dipping into red more his usefullness will be more limited, if he dies all that mana you had floating (that you could've used for other stuff) goes away, and he has no sort of evasion or protection of any kind. If you can't tell I'm not a really big fan. I think that in your deck Protean Hydra would be much more awesome. You can customize his size as he comes in based on how much mana you have available, so you can play him early if you need it, and his ability is just awesome giving him more size and survivability in combat and making him more of a threat as the game goes on.

Also I added Raging Ravine just because it gives you an additional way to smack your opponent and adds further mana fixing. Finally I'd also ask you to consider possibly running Harrow instead of Explore. The reasoning for this being that you aren't guaranteed that you'll draw and thus be able to play two lands when you cast it. With Harrow you know for a fact that two lands will hit the board that turn (and possibly a third if you had one to play already). This could potentially trigger the landfall ability on your geopedes and baloths 3 times in one turn which could be devastating. With explore you're only ever going to get 2 triggers.
 
So with the modifications we could be running a list that looks something like this:

18 Creautres

4x Llanowar Elves
2x Joraga Tree-Speaker
4x Plated Geopede
2x Protean Hyrda
2x Obstinate Baloth
2x Rampaging Baloths
2x Engulfing Slagwurm

18 Spells

4x Lightning Bolt
2x Fireball
2x Overwhelming Stampede
4x Cultivate
2x Garruk Wildspeaker
4x Explore

24 Land

12 Forests
6 Mountains 
4 Rootbound Crag
2 Raging Ravine

Sideboard

4x Plummet
3x Gravity Well
2x Steel Hellkite
2x Obstinate Baloth
4x Pryoclasm

As always do some play-testing to finalize your numbers.

-Dolf