Your journey into the mountains becomes harder with each step. As you approach a towering peak, the darkness grows into something almost tangible. The land tells you this is the source of the blight. You reach out to it, and suddenly your mind is overwhelmed by otherworldly visions of an ancient horror.
Your vision shows you strange beings attacking. Survive their assault to see the source of the corruption: the Eldrazi behemoth, Emrakul.
In this special duel, the opponent begins with 100 life and has Emrakul, the Aeons Torn among the seven cards in their opening hand. Apart from the usual win conditions, the duel also has a special win condition whereby Nissa wins when the opponent plays Emrakul, which costs 15 mana; essentially, this means Nissa wins when the opponent reaches 15 mana, since they always have Emrakul.
Visions of Emrakul has several creatures and other spells that create Eldrazi Spawn tokens, which can be sacrificed for burst mana; four Eldrazi Spawn can also be sacrificed as an alternative cost to cast Hand of Emrakul.
Since Nissa wins when Emrakul is played, the duel is, as the game informs you, simply a matter of surviving until the opponent reaches 15 mana (it's also possible to win via damage, but it's more difficult).
The only major threats the opponent has are the Hand of Emrakuls, which not only set your opponent back four mana from reaching Emrakul, but are also 7/7 creatures with annihilator 1. All of their other useful attackers are 2/2 or 3/3 creatures.
Scute Mob, if you manage to trigger it, is an amazing blocker. It can't be removed by a single Flame Slash after one trigger, and it overpowers a Hand of Emrakul after two.