Simply level up War Machine to level 60.
War Machine can be purchased from Adam Warlock for 400 Eternity Splinters.
Follow the following steps for the fastest way to level:
Lvl 1-9: Story missions or Midtown
Lvl 10-19: Industry City. Each event drops boxes that contain bonus XP. Make sure you kill or help with at least one the current objectives to get credit to get the box.
Lvl 20-60: Legendary Quests. Once you hit 20 you will be given a legendary quest to complete. This rewards you with an insane amount of XP. If a legendary quest pops up that is located in Story mode then spend the credits to skip the quest to get a new one that takes place in terminals or Midtown/Industry City/Uptown. The speed in which you'll earn more credits and XP outweigh the current price to skip for something better.