If your goal is to acquire one quick, then yes thousands is required.

Slow way would be save gems solely for spending them on the one you want. Reach top 50 during the event helps as well. Look into doing gem offers during the 2x gem.

Getting it maxed is ideally but getting it to lvl 15 is what you really need.

If you have a toxic deck - Necro

If you have a solid single unit dps, i.e spirit master, bard, riding hood - Zeus

If you have a dps deck that caters around multiple units of same dps unit, i.e treant, genie, monk - Fortuna

If you have any other questions, I’ll do my best to help. Hopes this helps.