The "choose a player" wording is very odd, but I guess it's so that you can choose whether you want to use this to attack or defend.

The "prevent that damage" is rather harsh. I guess it's fine if someone's winning with a creature swarm, but it seriously obstructs attempts to win with 1 or 2 big creatures or burn spells.

RE "choose a player": I wanted it to stay as a creature for a round, and transform when it got back to the player's turn who dealt the damage. I thought the way it is now would be easier to word and understand.

RE "prevent that damage": Will it prevent all damage done simultaneously, like a Fog effect during combat? Or can it trigger multiple times? I don't want it to be able to eliminate multiple sources at the same time.

Looks to me like it sets up a damage shield that prevents the next thing that hits you. I'm... not sure what happens from multiple combat damage; presumably the attacker chooses which one hits you 'first'. But possibly you get to choose. Either way, I think it needs clearing up on the card. It's a bigass rattlesnake, that's for sure. You could make 'choose a player' be 'current player'. But the current problem is "I don't CARE what the transformed side does, the main side means their biggest thing cannot hurt me. Ever." At least the transformed side isn't indestructible, just lifelinked, flying, firebreathing and and armoured. And oh my ... the transformed side is multiply lifelinked! Does that do what I fear it does?

Ha. Multiple lifelink no longer does what you fear it does. It did, for about six months, do that; known as the Genju of the Fields trick. But no, lifelink is redundant in multiples now, like flying.

And yeah, it'll only prevent damage from one attacker; if three 6/6s are hitting you, you'll choose one of them for this to absorb and you'll take the damage from the others. When you compare that to Fog Bank or Darksteel Myr or whatever, it's not exactly overpowered on a 6-mana rare.