At the beginning of your upkeep, Form of the Dragon deals 5 damage to target creature or player.
At the beginning of each end step, your life total becomes 5.
Creatures without flying can't attack you.
10/4/2004 If you life total was above 5 at end of turn, then you lose life to make your total 5. If it was less than 5, you gain life to bring it to 5.
10/4/2004 It sets your life total at the beginning of the end step of every player’s turn, not just your own.
2/1/2014 Unless some effect explicitly says otherwise, a creature that can’t attack you can still attack a planeswalker you control.