So I followed this guide. At first I chose Earth and it was long and tedious...as is this whole game.
But then I found this guide and switched to Fire and dropped my deck to 32 and it was amazing. I was practically winning every lvl3 match. So I had finally made enough money to buy the suggested Rainbow deck plus a couple upgrades, which I spent all my money on. I then proceeded to test it out on a few lvl3's, all of which beat me very quickly. I kept at this, thinking I just didn't have a feel for the deck yet. Nope. My win:lose ratio was about 1:10.
I too followed this (and other guides) and have "grinded" my way into two upgraded Rainbow decks and a partially upgraded Fire deck over the past week. I currently have a 16.5k score with 561 wins and 612 losses. I mostly fight FGs, with some farming of t50s.
Fire decks are great for speed and damage and they focus on winning in the early game; SG's rainbow deck is all about stalling and locking down the board to win in the end game (and you need the eternity weapon). It's hard to switch back and forth, especially if your own playstyle doesn't match a particular deck.
So I switch BACK to Fire to grind for more money to upgrade more cards. At this point my upgrades are all 4 Hourglasses and 1 Otyugh. Still, my winnings did not change. I don't see how I am supposed to beat a FG if I can't even beat a lvl3.
It isn't necessarily linear. AI3s don't have the same decks or playstyle as the FGs. Honestly, the Rainbow deck isn't really geared for farming AI3 (or t50, or AI5, for that matter). My preference is to use fire decks for farming those opponents. However, I found that forcing myself to play FG's with the unupgraded SG Rainbow deck, with a winning percentage starting at 0% rising to ~20% at best over 100 games. This really forces you to learn the deck although with a horrible win/loss record (if you care about those things). I went back and played against AI5s and saw where I could make mistakes and still recover and win against AI5s.
At this point, whichever Rainbow Deck I use I still only win about 40% of the time against FGs, but it is getting faster (not easier) with the FG farming. And, I am still on the steep part of the learning curve
IMHO, for a game as repetitive as this, there is far too much grinding necessary to get to the kinda good stuff. There should be different prices for certain cards to be upgraded. For instance, a pillar should not be $1500.
I had more to say concerning upgrades but I figured I'll get ripped apart for this post anyway, so why say it.
Actually, IMHO, it's easier (more enjoyable) to farm the FGs (or t50) for extra cards and sell those, than it is to "grind" money. Plus, if you win cards from FGs, they are fully upgraded, so you can either use them right away, or sell them for ~1100-1200 a piece.
If you follow SG's unupgraded (almost) rainbow thread, I would recommend reading the comments by jallenw, as those seemed to help me the most in understanding how to progress with SG's rainbow deck.