I read a post by a newbie saying how he cant wait to upgrade a particular card to use in his fun PvP deck and I noticed that it is one of the many <read title> so I came up with a list based on my own experience grinding FGs using all sorts of rainbow and other anti-FG decks including RoL/Hope, pesty fractal, nymphomania etc :-
Supernova
Graveyard
Bonewall
Retrovirus
Unstoppable
Elite Graboid
Feral Bond
Fire Lance
Explosion
Abyss Crawler
Arctic Octopus
Congeal
Ray of Light
Improved Blessing
Improved Miracle
Animate Weapon
Elite Queen
Eagle's Eye
Unstable Gas
Shockwave
Electrum Hourglass
Pharaoh
Rewind
Pest
Siphon Life
Vampire
Twin Universe
Phase Shield
Fractal
you may argue winning any cards (pre-updates) vs FGs but this is a most likely list due to a couple of reasons :-
1) FG difficulty (it is harder to win cards from harder FGs i.e. Rainbow, Hermes, Divine Glory etc)
2) # of a particular card ratio to the whole FG deck (it is harder to win cards from a varied deck i.e. Rainbow, Dream Catcher, Chaos Lord etc)
3) # of a particular card ratio to the whole FGs total (Twin Universe, Animate Weapon, Feral Bond is in more decklist than any other non-pillar cards - for example)
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let me know if I miss any
i didnt include Quicksand (for example) because not only Seism is a somewhat difficult FG, but while you can also get it from other FG (Dream Catcher) the ratio is too small and it is also harder to win in spin vs Dream Catcher for reason #1 and #2