You lose 4 damage minimum, not 8 and pay an awful lot less than you would for golems or dragons. A creature that takes 2 spells minimum to kill is good whether it has 8 HP or 5.
Once again, the problem is not with an 8 HP phoenix for 7, that's fine, but with a 5 HP minor phoenix for 2.
2 mana for four damage a turn unless your opponent has 2 cards to deal with it is hugely powerful already. Making it almost totally immune to the second card just makes that all the worse. If you really want your minor phoenix as is, would you pay 3 for it?