Key cards of FG Decks are good for first upgrade, like Ray of Light , SoG, Supernova, Hourglass...
RoL - Hope is a good starter for fights against FGs. I wouldn´t completely upgrade it (better decks exist)
I would go to the wiki and look through the list of cards and their upgrades, you´ll probably find some interesting and some uninteresting upgrades.
Actually, for SPEED GRINDING, the best is RoL/Hope. It beats some gods easily, and there are a few you must skip, leading to short matches if you lose, and long ones only if you win (and against some pesky gods).
Every other FG grinding deck (I mean Rainbows such as CCYB) can beat any FG, thus you will lose lots of time being beaten by most of them (it can lose to any god with a bad draw, and bad draws are far more common in this decks).
In average, I guess RoL/Hope is the fastest electrum grinder out there. I tried lots of decks (CCYB for months, Pestal, Oty-Bonewall-Druid variations) and RoL/Hope is my favourite by far.
First upgrade the RoLs, or you won't be able to play. Then Electrocutors, then Fractal/Aether pillars and Dragons last. Don't upgrade Hope, since it costs you 1
![Light :light](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/light18x18.png)
more, and that's generally the difference between being dead and slaying a FG.