Flame Sovereign, Rare, 30g 15r
Burning. Mage
Attack: M*N/2
(max 80)
M: # of Searing Fires in game
N: # of gems
I can't help comparing this to Phoenix since their costs and variable damage are very similar. In comparison, this one costs a bit more and does less, and this doesn't stay on hand or return gems or create more Burning cards for subsequent plays as Phoenix does when properly used.
I realize that there are definitely cases where this would do more damage since you can derive it from the gem value rather than the number of (one particular) Burning, but if you have enough gems for this to be superior with its one attack, Phoenix would still be a better option for its second strike, assuming you started with 17* recruits rather than 15 (with standard Dungeon values, of course.) That is, since you necessarily have the gems for it in these specific cases.
* 19 if you can expect your opponent to use Searing fire. Just covering my bases here. ;)
TLDR version: Costs more than Phoenix, does less.