When you have beaten the first few levels of Story mode you will unlock the Daily Challenge. This feature is basically an endless run where you have to kill a certain number of zombies to ean different tiers of rewards. A Bronze score will earn you Silver, a Silver score will get you gold and a Gold score will give you weapon parts. You do not have to expend a type of currency in Daily Challenge mode so take advantage of it and try to get a Gold score and a decent stash of weapons parts.