King of the Mountain/Walkthrough

Step One
You'll begin the first story mission by entering a mountainous area and, under Lord Saladin's instructions, making your way to a gondola that takes you up toward Felwinter Peak, where a Fallen incursion has been detected. Fallen are everywhere along the way, shooting at you in the gondola, so keep your eyes up! Scout Rifles and Snipers, are definitely recommended here as there's a lot of longer range fighting taking place. Eventually a Fallen ship blows a hole in the gondola and you have to jump out and make the rest of the way up on foot to the mountaintop before having to defend a courtyard outside the entrance to the Iron Temple from several waves of Fallen.

Step 2
After fighting off the enemy waves, you encounter Sepiks Perfected, whom you must prevent from gaining access to Felwinter Peak. As Ghost observes, Sepiks is regenerating due to the SIVA influence, and maintains a triple burst shot so beware. He will have a shield that makes him invincible, but it will disappear on its own after a while. Completion of this portion of the level grants you a Grimoire card and access to the Temple.