Too little creatures. Yes, I know its the theme, but the three devourers do not damage at all and the lava golem and forest spirit are both very vulnerable to cc. It looks a bit like an unupped version of the shutdown, which you can look at for some ideas (
http://elementscommunity.org/forum/index.php/topic,8420.0.html). I would replace the purify and maybe a fog shield for more creatures.
I don't get your point about the arsenic though. Yes, it is better upgraded. Most cards are better upgraded, which is why people actually spend
to upgrade cards. I would actually prioritise upgrading the forest spirit and novae over arsenic so that the forest spirit might actually survive the first few turns and to speed up the deck. Seeing as how this is a pvp deck though, you are probably playing in pvp 1 and upgraded cards will be turned back to unupgraded cards anyway.