What to do for mid-level players?

[Opsinis]

I'm no where near a status of money-saving. Honestly, I sell rares as soon as I get them. I keep my entire card collection to a neat 6/6 or 12 (depending on upped or not). It's a compulsive thing. Kinda nice, kinda not. The max amount of electrum I accumulate is about 40k then I will either work toward completing an upped set of an element OR I just up pillars/pends. I really love Towers, they're so shiny. It a compulsive thing. :/

[neuroleptics]

what i would suggest is
1) you start by upgrading your favourite element. I got  upgraded 1st. That helped in pvp events apart from tourney.
2) If not, start by buying all unups tho i would suggest you buy at least 3 per card and have at least 3 upgrades per cards. That's what i did (since you mention the aim is to get 12; 6 ups and 6 non.)
3) Even so, if you plan to play casually and have fun, you could look into deck section, look for interesting decks, those that have provided good stats against FG or pvp, start by upgrading the entire new deck and toy around with it.

>>>But of course, make sure you have enough  left. I did the same, grinding and earning enough upgrades for two decks (60 cards =1500 x 60) then i upgrade a deck and the cycle repeats.

>>>Keeping it 12 (6 ups and 6 non) is unnecessary at this point since you will get lots of them again when you farm. The few electrum sold doesn't matter since you have sufficient  isn't it?  You can just sell it after you complete a few elements, then given the amount of extra cards, you could probably buy a few upgraded cards to complete a trainer edition at the end of day

[Keeps]

I like to skim through the forums and find neat decks I haven't thought of that I'm missing just a couple of the upgraded cards on, then upgrade those cards so I can say that's another deck I can build. 

When I can't find any decks like that, I put money into towers and pends