Alright, so while some people may go define grinding on a dictionary, I'm using it in the sense of repeatedly playing against something.
We're not talking about skateboards nor wood/metal working. What 'grinding' is, in Elements, is repeatedly playing against a certain type of AI (Level 0 [AI0], Level 1[AI1], Level 2 [AI2], Level 3 [AI3], Top 50 [AI4], Level 5 [AI5], and Fake God [FG]) The Level of the computer is typically refered to by AI (#). Some people will refer to Level 3 as Elder(s) or AI3, and Level 5 as AI5 or Halfblood. Fake God is also referred to as "FG".
Now that we're past that we can move to grinding. Grinding is, typically, playing against Level 3 (from now on referred to as AI3), Halfblood (now on, AI5), or Fake God (now FG) repeatedly to gain score, electrum (money) or just to have a certain number of wins or to test a deck.
Grinding Ai3
Grinding (repeatedly playing against) AI3 is typically considered the best for gaining score rapidly. Winning with an Elemental Mastery (EM; winning with full health) against an AI3 gives you 35 score and 40 coins + spins. Most decks for score are made and tested against AI3. These decks will give you a fast win rate and also a high Elemental Mastery rate.
Grinding Ai5
Grinding against AI5 is best if you aren't able to make a deck that can beat Fake Gods but you can easily beat AI3. AI5 decks are made from 2 elements, randomly, and there are 144 different duo's (duo = two. duo in this means deck with two different elements), but have a random assortment of cards. AI5 decks have 30% upgraded cards, meaning that you have a better chance to win them than before v1.2.6 of Elements, but still it's not a super high chance to win upgraded cards. These upgraded cards sell for ~1100 electrum, worth the time to grind for them.
Grinding Fake Gods
Grinding Fake Gods is almost always to get electrum. Because cards won from FG's are always upgraded, they are worth ~1100 electrum per card. There are many different FG decks out there, all intended to beat FG's, but some to do it slightly differently. CCYB; Creature Control Why Bother, is a very popular deck that can beat the majority of Fake Gods while leaving you with close to enough health to get an Elemental Mastery. Shak'ars Revenge is fast and can beat most Fake Gods, but will not be anywhere close to an Elemental Mastery. RoL Hope; Ray of Light Hope; is a cheap deck that requires only 6 upgrades to build, but does not have as good of a win percentage.
OTHER THINGS:
Grinding anything besides AI3 AI5 or Fake Gods is nearly worthless, although t50, which is made of the decks of the top 50 players, played by a computer, will often have rares in the decks, it's very unpredictable.