The first step to complete this is finishing the campaign. Then you can go to
Bonuses and
Extra Content Shop in the main menu. Here you'll be able to buy infinite ammo for the GM 79 (or another one of your favorite weapons).
Now start New Game+ on Casual, and proceed for about 10 minutes until the first Lycan attack and Urias will eventually spawn. Once he spawns, you should have about 3 minutes before you fail the section as a part of the story. Use your infinite ammo GM 79 until Urias falls and you're done!