The CLEO heavy Sniper Rifle is the second to last item you unlock. You can only unlock this by finishing wave 7 many times. After you unlock the rifle in the Daybreak DLC, you will need to return to the main game and kill a screamer with the weapon. The best place to find them is at the infected houses.
How to Unlock
At first, you will get items for completing waves 3, 5 and 7. However, as you advance, you will stop receiving items for waves 3 and 5. That means, if you keep failing without completing wave 7 you will have a long road ahead.
I am not sure about the exact count, but you will need to complete around 17-22 complete games to acquire the CLEO sniper rifle. I was pretty successful with always completing all waves, and when wave 5 no longer gave me an item, I still had 7 items to unlock.
It should also be noted, that if you join a game after wave 3 has been completed, you WILL NOT GET CREDIT for completing 7 waves. But if you join (including loading in) seconds before they complete wave 3, you will get credit (if you complete wave 7) (I joined at 22 seconds to the end of wave 3 and got credit).