Commander - Xira Arien

So trying to fine-tune an EDH deck is admittedly harder than a normal standard deck because you're going to experience a pretty large amount of variation across the draws and how the deck will perform.

That being said, I think the biggest issue I see here comes from the lack of board wipes at your disposal.  Sure, you have Decree of Pain and Plague Wind, but those are LATE game wipes.  Earthquake, Magmaquake, Savage Twister, Mutilate, Damnation, Gaze of Granite, Mizzium Mortars, and even Inferno could help here.  At the end of the day, you're likely going to be sitting on a singular more dangerous threat than most of the opponents and, as such, you want to ensure that said threat is the only thing that stays on the field.

I'm also a fan of adding Fog-esque effects in most EDH decks as I find that 4-player games typically sit with a stalled board state until someone has the ability to alpha strike the entire table.  Being able to negate that attack often will allow you to simply win on the crack-back.

A few things about your specific build I'm thinking you might benefit from:

Triumph of Ferocity - if your guys are bigger on average, this essentially equates into card advantage every turn.  Why not?

Huntmaster of the Fells - why not?  You're in the colors, they're a LOT cheaper than when they were standard legal, and it can do some work for ya.

Whip of Erebos - lifelink for all your creatures?  This is huge in ANY format.

Giant Adephage/Utvara Hellkite - they're expensive, but they're huge and can easily win a game all on their own.

Any of the other walkers would be good, in particular, Xenagos has some promise in EDH given that he helps with ramping or at least can spit out some blockers in a pinch.

Lastly, take a gander at the pre-made Jund Commander 2013 decklist (Prossh if I'm correct) as Wizards was actually pretty good at putting some decent stuff in the commander decks this go 'round - many people I know haven't even really modified them beyond how they came.

I hope some of this is helpful... I guess a lot depends on how competitive your EDH players are (obviously those wonderful Spike players will seek to find broken combos and will ruin the game for everyone no matter what you do).