NOTE: FOR THIS GUIDE YOU HAVE TO FINISH THE GAME AND SAVE YOUR COMPLETED DATA WITH THE GRENADE LAUNCHER IN YOUR INVENTORY
This is easiest to do on casual difficulty with the infinite ammo grenade launcher, as you will be able to purchase it from the bonus item shop as soon as you beat the game one time. Once you do that, load up your completed save to start new game plus, so you will have all previous guns from your last run. Simply play through the game until you get to the Lycan attack, and survive until Urias shows up. Then continuously blast him with it until he dies.
(This works even better with the hand cannon, as he dies in about 3 shots from that on casual, but if you don't have ammo for it, it takes a lot longer to get the infinite ammo version)