I used to play this game a few years ago, and I thought it was impossible. Even AI3 was a too much for me. But after coming back to it, doing a little bit of research as to what Elements are best for what, what kind of strategies exist, and imitating decks found from lurking, I do alright. I mostly focus on mono and duo decks, though I have attempted rainbows on occasion.
Elements is certainly my favorite online CCG, due principally to the wide array of viable strategies and synergies that can be found. It also helps that there are no micro-transactions
Its very rare to find free games with this level of complexity and replayability.
In terms of favorite elements, I like most of them, but tend to favor Aether, Death, Darkness, and Time.