Phoenix is 7atk (6 cost) + 1hp (0 cost) + phoenix (1 cost) = 7
Immortal is immortal 4 attack (4*1.5=6 cost) + 3hp (0 cost) = 6
Minor Phoenix is 4atk (3 cost) + 1hp (0 csot) + phoenix (1 cost) - Upgrade (2 cost) = 2
Elite Immortal is immortal 5atk (5*1.5=7.5) + 4hp (0 cost) - Upgrade (0.5) = 7
Elite immortal may be a bit UP since it's upgrade bonus is less than 1. A dead phoenix requires 1 turn + 1 to bring back while immortals can't be targetted at all. Immortals suffer less from debilitating shields than fire's other creatures.
You are assuming Immaterial should worth the *1.5, but my point is they aren't.
Or you are assuming Phoenix(Skill) should only worth 1
, but how can that be, if Immaterial worth that whole *1.5?
For damaging spell, when you put the fact that Phoenix are almost triple in attack for same resource in consideration, you will find that single target spells never pull Phoenix to Immortal's pathetic amount of damage.
Most mass damaging spell won't do their job well, if you consider their cost.
The only spell that Immortal do better than Phoenix is Thunderstorm, if your own field aren't considered, Pandemonium.
For debilitating shields, fire is one of the best elements against them, just get rid with 2
...
For now the only point to using Immortal are SoW, which are petty pathetic...