This can be done on Easy. Steal Patriot's uniform, enter the ring with Sanchez, Kill him in a quicktime event.
On Easy, It's simple to kill all the enemies in first area with your pistol. On harder difficulties, You'll have to sneak around to the Punching bag, Patriot is the one hitting it. Then go up to the righthand door, the one that doesn't need a keycard.
Sanchez is simple to defeat. Just follow the on-screen prompts. It's a quick-time event. .