Your mono green is weak

t1 arbor - llanowar t2 archdruid - baloth - channelers - boa t3 gigantiform - troll - wolfbriar t4 overwhelming stampede obviously you have multiple options on the t3 and t4 with 5 or 6 mana available. being that uw has the most threats to the deck a large portion of the sb is set for that. vines of vastwood in the main helps to protect vs path to exile - codemn. regenerate creatures are there for excellent blockers, mass removal, and destroy effects. boa also does well vs jtms. gigantiform is in the deck because i find its more likely to deal quick damage unlike beastmasters - eldrazi monument. though primal bellow does look really good, i dont think its worth 8 like gig on t3-4. though the cheaper cost is nice, it unlike gig does not stick. elfs are mainly in there for mana production - creatures for stampede. even without stampede this deck can consistently deal 20 on t5-6. i would love to hear your input. i will be playing testing this.