The goal of this deck is to compound elves to produce obnoxious amounts of mana, which it does shockingly well. The biggest weakness is that it focuses on direct attacks and defense, meaning it works best against decks that do the same.
Step 1: The main objective at the start of the game is to increase mana production. Elvish Archdruid, Marwyn the Nurturer, Priest of Titania and Rofellos Llanowar Emissary all help to do this in slightly different ways.
Getting out Marwyn the Nurturer as soon as possible is highly helpful, since it is the only card that improves based on how many Elves enter the battlefield.
Rofellos Llanowar Emissary is also a good starting creature card, since it produces mana based on number of forests, meaning you will likely get a positive mana return immediately.
Priest of Titania has the weakness that it only yields mana equal to the number of Elves on the battlefield, making it less than optimal as a first creature card, but quickly useful as more Elves are played.
Elvish Archdruid may actually be the last to be played, since it is essentially a version of Priest of Titania that costs one more mana. It is extremely useful to have out not just because it increases the power and toughness of other Elves, but because that increase makes Marwyn the Nurturer produce more mana.
Step 2: Once sufficient mana is being produced, the priority is not to loose it. Omnath Locus of Mana allows you to store your mana, building it to ridiculous levels over multiple turns.
Since Omnath Locus of Mana increases in power and toughness based on your unspent mana, it will become a powerful offensive and defensive tool in addition to helping you retain your mana.
Step 3: Now that you have mana, you need to use it. The most straightforward use is to leave it to empower Omnath Locus of Mana. If you put Helm of the Host on Omnath Locus of Mana, its copies will also gain +1/+1 for each unspent mana you possess, giving you an increasing number of powerful creatures.
Alternatively, Oracle of Nectars can be used to convert your mana into an equal amount of health. I find this typically gives me over a hundred health at this point, which is a comfortable buffer against most damage based decks.
Lifeblood Hydra can also be a useful way to spend your mana, since it quickly gives you a powerful creature. It also has the side benefit that opponents will often be reluctant to destroy Lifeblood Hydra, since its destruction will give you access to enough cards to greatly increase your resources.
Emrakul, the Aeons Torn: Emrakul the Aeons Torn is included mostly to deal with certain situations and because its mana cost is usually easy to pay. It is most often useful as a response to card combinations that make an opponent hard to deal with directly.
Since it forces the opponent to sacrifice 6 permanents, most decks will be set back severely. Simultaneously, Emrakul the Aeons Torn provides an extra turn to gain the upper hand and cannot be countered. This is all on top of it being a relatively strong creature.
A less apparent reason for it to be included is the protection from milling to death it provides. Since it is reshuffled into the library instead of entering the graveyard, it stops you from milling to death when Lifeblood Hydra is killed.
