There are several methods on youtube, but in this guide I'll show you the ones I use that can help build/improve your weapons.

The tips are:

1. Earn gold by watching ads or taking daily challenges to buy an item that doubles the money you earn.
2. Play in the challenges that kill more zombies and collecting secrets (Golden Pigs), for example: Sentinel, Assault or play in the Suburban Ride (Europe) stage.
3. Challenges earn you 2x or 3x money.

.

Image 1:

.

Image 2:

.

Video:

.

Video 2: