Standard: GW, Tokens/life . . . Needs focus

The local environment (given my limited exposure) leans more toward the budget and home-brews.

Fleecemanes would definately help protect from removal . . .

I appreciate the advice

The question is what exactly you would need Pacifism for.  It's solid added removal, but is there a particular target you had in mind?  Some creature that's causing you problems otherwise?  If you're having issues where you need more removal, what types of stuff are you looking at?  If it's just little aggro stuff overwhelming you, Last Breath and a variety of fight stuff is useful.  If it's bigger things, Selesnya Charm and then Pacifism might be a good option. 

Simply put, sideboard cards need to be cards that you can easily imagine bringing in for a specific problem card/cards.  If you don't know what the card is an answer for, then you don't want/need it in the SB.  If it's a card you're dying to put in the SB because it has tons of applications for your deck, you might consider putting it in the MB.

Simply put, flooding the board.  If you can resolve more threats than they can shoot out removal spells, you have a chance.  Typically mono black gets hammered pretty hard by aggressive decks as it normally takes them a while to stabilize the board.

So think low CMC critters that pressure them.... double up on one-drops and two-drops as every time they have to spend three to Hero's Downfall your one or two drop, you're gaining advantage in terms of mana spent, and, in turn, tempo...

If the link is still the same as in the OP with the updated build I'll take another look to see if there's anything that can help in that matchup.

So against Mono-black you're going to want to move some things around...

First off, the SB doesn't really strike me as being put together for the purpose of facing specific match-ups.  Often times people will fall victim to simply putting cards 61-75 that they would want to mainboard into the sideboard, which, unfortunately, is not the way to use a SB.  The SB should be 15 cards that are very situational and useful in particular match-ups.  For instance, Pithing Needle is a great SB card at the moment as it can be brought in game 2 to shut down PWs, Underworld Connections (you name Swamp), troublesome monstrous guys (think Polukranos or Stormbreath Dragon), stop someone from using Ratchet Bomb to hose you, etc.  Now keep in mind this is a card you'd normally never put into the mainboard of your deck, but against Mono-Black, esper control, U/W control, and G/R Devotion, this can be a HUGE play.

Looking at your SB, I'm seeing a lot of sub-optimal cards.  1x Giant Growth?  If you want the full suite of four, why not just find room in the mainboard?  Savage Surge?  1x Path of Bravery? 1x Trostani?  2x Angelic Accord?  A lot of these strike me as being cards you'd want either in the main or not at all.  You need to be able to identify specific cards you'd see from an opponent to bring these in...

So what cards do you need to look toward? 

4x Selesnya Charm should be in here.  I'd say to do them over Giant Growth - sure, it's one more to cast and one less pump, but it exiles big threats (see Thassa, God of the Sea and Desecration Demon), pumps while giving trample (definitely big if you do it on Fabled Hero as the second strike will deal all to the opponent care of trample), or can crap out a 2/2 guy for ya.

Gods Willing should be in some higher number.  If anything, I would move Rootborn Defenses to the SB as the spot protection is likely going to help you more frequently than the mass protection (I get the loss of populate sucks, but if you put the second Trostani into the main, you'll have all the populate you could ever want).  It offers a heroic trigger AND can protect your guys in a spot (nevermind giving a Fabled Hero a free swing).

I would suggest finding a way to move the count of Soldier of the Pantheon up to 4x (or at least include the other two in the SB).  This lets you side in a few more one-drops to make your deck that little bit faster against Mono-Black or other control match-ups.

Last Breath should be in the SB in some quantity if you were to encounter Mono-Blue at any point as it's one of the only options you have to remove Master of Waves which, otherwise, would just beat you as soon as it hits the field.

You're close to a really solid G/W build, hopefully these comments will help.  Remember to always be thinking about which cards are performing well and which are underperforming.  Is there a card you're always sideboarding out?  How about a card you're always sideboarding in?  If this is the case, you might want to reconsider what goes in the MB and what's in the SB.  Are there cards you're NEVER sideboarding in?  If so, do you really need to use those spots for them?  There's always room for improvement both in terms of play and build, so keep your eyes open.  And remember, this deck is only likely to get better with Born of the Gods as they'll release the G/W scry land and such...

There is a guy that runs a very similar Selesnya aggro type deck at my local FNM and he does quite well with it...He maindecks Experiment One to get a fast growing board presence against devotion decks(Primarily mono black) and often times is sitting there on turn 2 with a 2/2 Experiment One and a 3/3 Fleecemane out there already...I would highly recommend Selesnya Charm as it can remove things cheaply like Des demons and it serves double duty in giving you a pump spell that is similar to Giant Growth, just slightly smaller but w/ trample....and I guess if you really had to...throw a knight token out there then a populate from trostani...gain 4 and a quick set of 2/2's in a pinch if you can't remove it with the charm.