So with that restriction you have its a bit hard to give helpful criticism. I will suggest cards but I do not know what you have and don't have.
Some cards that I think would help instantly would be
Lightning Greaves and
Vines of Vastwood. Soren is correct in that
Elvish Piper will feel the sunshine on the battlefield and then instantly be 6 feet under. These cards can help keep him a live a little longer. You also need to know if you want to cheat creatures into play or cast big creatures.
With Green Fatty Ramp, there is usually a handful of fatties to drop into play. The number you have needs to be trimmed a bit. 2 Emrakuls I feel is ok, but you have to cast Emrakul in order to really make him worth it.
One thing I usually try for with big creatures are the come into play abilities over attack abilities. This way you are spending the mana and getting instant results.
Bane of Bala Ged,
Breaker of Armies, and
Conduit of Ruin could be better creatures. With mono green I love me some
Sundering Titan. Other good creatures could include:
Thragtusk,
Acidic Slime, and
Terastodon.