I think it's a good idea to keep the phase dragon immortal without alternation.
Aether is not really weak against gravity, my immortals and my shields work fine, momentum can be a problem, but most of the times I have Lightning or parallel universe ready to take advantage of that, Lobotomizer works great too.
Weakness of Aether is Darkness (like most other decks I guess) since steal is a real problem for a deck depending on it's shields. Also the devourers are draining to much energy for a expensive element like Aether.
For the same reason fire is quite strong against Aether, destroying the shields is not as bad as stealing them for the Aether Players, but without the defence, Aether can only hope to have enough Lightning to reduce the attackers forces. In most cases you can't outdamage them with your creatures.
There are a few other combinations which can be nasty, more than gravity at least, are you sure Aether should be weak against it?
