I found a method where you can get so much gems quickly and it is kind of simple (also credit to the person i credited)
I call this trick "Replaying Campaign" because all you have to do just replaying campaign. Why? Well let me explain. (Sorry if you all don't understand it's to me to explain it since im not that good at english)
When you play a campaign, there will like 6 bonus stages marked as stars, right? Each stages if you win you will get rewards like spells or skins. When the first time playing, you just get the skins since you don't have it at first. But this is where trick is. The skins. When you already have a skin and you get the same skin, you will get some gems as the compensation, right? Well it also happens when you got skins from the bonus campaign. The best stage is, is the 6 stage where it give you a Vampiric Swordwrath skin. When you already had a Vampiric skin, it will give you like 1000 gems as the compensation. This is where the trick is again. When you delete and made a new save file, if you replay the bonus stages and win it, you still get the rewards from the bonus stages! Which mean yeah, you just need to play a bonus stages where it rewarded the skins, it will compensation you with gems! Since you already got them in your first playthrough. So yeah this trick only works if you are veteran and not a REALLY REALLY new player. Yeah i hope you all understand my explanations since i really don't how to explains it if you all don't understand, my bad :(