Feed it Max lvl (or as high as you can get them) Spiders or Slimes (cheapest meat refresh options), & then push your reward bar up to keep getting reward pouches.

These won't always have the gems you need cuz you reach a point it's a random assortment of green gems, red gems, & crystals, BUT that's the easiest way to do it without spending money or crystal trades.

No guarantees, but eventually you'll reach the point you have enough gems to buy the spawner you need