Well, Scott, let me just thank you for coming to me for help.  I'm quite flattered, and I'll be happy to assist in any way.

 

Now, you said the focus of your deck is mostly Library of Leng/Multani, Maro-Sorcerer, plus lots of big creatures.  The trouble is, you only have one copy each of Library of Leng and Multani, Maro-Sorcerer.  A general rule of thumb is that if your deck is built around a card, you need four copies of it.  I know it can be hard to get four copies of older cards like that, but usually you can use other cards with similar effects.  Library of Leng is similar to Spellbook or Graceful Adept, and Multani is similar to Maro, Masumaro, First to Live, and Soramaro, First to Dream.  Basically a Gatherer search for "maro" will turn up plenty of creatures that would go great in your deck.  In the meantime, I'll keep the focus of the deck "big creatures-handful of cards".  Green is good at big creatures and Blue is good at drawing cards, so this shouldn't be too hard. 

 

I also noticed that you have 15 cards devoted to life gain and holding off your opponent. This tells me you're having trouble staying alive long enough to get your big creatures out.  The solution is that your deck isn't fast enough.  Green decks normally get their speed from having more mana than just one land per turn, with cards like Llanowar Elves, Sakura-Tribe Elder, and Kodama's Reach.  Any of these cards would help you play your expensive spells earlier.  Blue, on the other hand, likes to draw lots of cards, and with Multani in your deck, you should be drawing cards like crazy.  Right now, Omen and Opt are the only things really helping you in that respect, so I would advise getting cards that draw cards, like Telling Time, Tidings, Petals of Insight, and Lore Broker.  Lore Broker especially combos nicely with Library of Leng, since your opponent will have to discard while you can just put a card on top of your library.  There’s also Jungle Barrier, a green and blue Wall that draws you a card when you play it.  My very first deck was green/blue, and used Jungle Barrier quite well, so I know what I’m talking about.  Once you can reliably fill your hand with cards, you’ll be much more likely to have the card you need when you need it.

 

Also, you should realize that having only one copy of a card is a bad idea.  In a typical game, you only draw about half your deck.  If you only have one copy of a card, you will only draw it in half your games.  And even in some of those games, it may be too late or you may not even need it.  As far as making room for the cards I’ve suggested goes, I suggest you take a hard look at each card you only have one of and ask yourself: is it worth adding more copies?  If so, then do that.  Otherwise, it probably isn’t good enough for your deck, unless of course you can “tutor” for it.

 

“Tutors” are just cards that let you search your library for another card.  You can get away with only having one copy of a card if you have ways of searching for it, since the tutor card essentially acts as a second copy of the card.  Most tutors, however, restrict what type of card you can get with them.  Green, for example is good at tutoring for lands but not much else.  The only card in your deck I see worth adding tutors for is Multani, Maro-Sorcerer, and I have just the tutor to search for it: Time of Need, which searches for a legendary creature card.  If you had a few of those, you could add a few more big nasty legendary creatures, like Masumaro and Soramaro.  Also consider Seek the Horizon, which searches for three lands.  Putting three lands in your hand would help Multani and the Maros(if you add them), plus give you plenty of land.  Searching for land is better than most people think: the more land you get out of your deck, the less likely you’ll draw one later when you need a spell.

Also, as far as your “pump up” cards go, you’re definitely on the right track.  Monstrous Growth isn’t nearly as good as Giant Growth, simply because “+X/+X until end of turn” should always be an instant, that way you can wait until your opponent blocks and then decide which creature to pump up.  Yavimaya’s Embrace overcomes this because it steals a creature, and Elephant Guide does give you a “second chance”, so that’s all right too I suppose.

 

That should be plenty for you to think about, so I’ll wrap up here.  I’ve listed a lot of cards for you to consider adding, but please realize I don’t mean you should add ALL of them, I’m just trying to provide options.  You might find better cards that I’ve forgotten, and that’s okay too because it’s your deck and you have to play the cards you like.  You can find plenty more options at gatherer.wizards.com, a comprehensive card database containing every Magic card ever printed, with a search function.  Thanks again for letting me share in the fun of building your deck, good luck to you, and happy holidays.  I’ll end by adding some comments to each card in the decklist you sent, just to make clear which cards would best be dropped.
 

Overrun Deck:

 

Land: 11 forest

10 island

 

Creatures:

 

1x Muktani, Maro Sorcerer

3x Gaea's Skyfolk(good, hard to block, easy to pump up)

3x Glade Gnarr(could be good if you’ve got plenty of blue instants)

2x Penumbra Kavu(good)

3x Penumbra Bobcat(good)

1x Gorilla Titan(I guess if you can keep your graveyard empty, but there’s better cards)

1x Scaled Wurm(BIG, but not much else.  Give him trample or flying and then we’ll talk)

1x Vizzerdrix(not worth the mana)

 

Pump Up Cards:

1x Yamimaya's Embrace(8 mana is a lot, but it’s worth it)

3x Monstrous Growth(see above)

1x Giant Growth(excellent)

2x Elephant Guide(good)

1x Shimmering Wings(I’m underwhelmed)

 

Life Gain Cards:

3x Blossoming Wreath(This is at odds with gorilla titan.  It is very cheap though, so...)

1x Soul Net(not too bad, but life gain as a rule isn’t that great anyway)

1x Wooden Sphere(you want to get this deck to the point where it doesn’t need life gain)

 

Survival/Hold Off Cards:

 

1x Regeneration(meh)

1x Opt(nice)

2x Omen(nice)

1x Bamboozle(meh)

1x Tangle(this one’s pretty darn good, actually)

1x Boomerang(you definitely want more of these)

2x Fog(very nice)

1x Rhystic Deluge(rule of thumb: all the ‘rhystic’ cards are lousy.  This one’s hardly an exception)

 

Main Focus Card:

Library of Leng