I was hoping that there was a little more content with regards to Arena Bo1 here, but I guess not. I wouldn't say I'm a great (and sometimes not even good) Magic player, but Arena, with the quick games, has gotten me practiced with certain aspects of my game. Things like when and how aggressively to mulligan, when to concede, and remembering combos have really improved in my game.
I haven't spent any real money in Arena, and have only improved my deck via spending in-game earnings. My favourite deck right now is:
Okay, that was tougher than I thought, making the cuts, phew. Thrashing brontadon is good still because it kills embercleave so if you keep losing to it put it back in. There is plenty of great 2 mana creatures that care about +1/+1 counters and creature size like Nessian Hornbeetle and Drowsing Tyrannodon and scavenging ooze (if you need grave hate). I still like Nylea it is just that I put in a lot of multiple green cards and the mana cost reduction is less, but you can easily make her a creature this way.