For this evaluation, we graduate from the common cards to two Rare cards from the Fire Splinter, the FIRE ELEMENTAL and the LIVING LAVA.
First up we have the FIRE ELEMENTAL, a rare ranged monster who's closest competitor is the epic FIRE DEMON from the Beta Edition cards. Is it better though?
Like the FIRE DEMON has at level 3, the FIRE ELEMENTAL has the Blast ability right away. For the extra 1 mana cost, you get the Affliction ability at level 5, and the Redemption ability when the card is maxed out. While a maxed out card is quicker, it also does 1 less damage than the FIRE DEMON.
I don't think this card will outperform the FIRE DEMON in lower mana rounds until it gets maxed out. At that point you can place it in the first ranged position as sniper bait, which will trigger the Redemption ability when he dies. For that purpose, it makes sense to keep this card, but I'd rather max out my FIRE DEMON first (a few cards short). Given my list of cards I want to upgrade, it may be a while.
Verdict: Keep and eventually upgrade. Sell the gold foil.
Next in the 'flow', we have the LIVING LAVA, a rare tank monster. His closest competitor from the Beta edition is the SERPENTINE SOLDIER.
For this guy, the story is pretty much the same as the FIRE ELEMENTAL when comparing it to its Beta counterpart, except the LIVING LAVA costs 2 more mana than the SERPENTINE SOLDIER. Instead of the Dodge ability, he has the Rust ability, which reduces the opponents armor. At max level, he does more damage and gets the Thorns ability, but never has a speed faster than 1.
When we combine the 35 cards together, we get 2/3 of the way to level 5, and the rust ability.
For this guy, I think there are definitely going to be some niche uses, such as Reverse Speed high mana rounds. Beyond that there are better tanks available for the Fire splinter in my opinion.
Verdict: Keep and eventually upgrade, but no hurry. Sell the gold foil.