sell some upgraded cards and buy unupgraded cards for unupgraded decks (I assume, you don´t have all bazaar cards). Don´t sell the arsenic card.
With your rares, I would suggest a water deck (or water/death with squid+ arsenic+ Abyss Crawler)
Or mono-aether (or aether+ fire mark + deflag). Mono-aether is quite good against T50/Lvl5