+1 vote for a rename to Tortoise.
I think Attack and Damage are high on both accounts. I would make the normal 4|4 and the upped 5|5, and I think that fits better with its cost.
If you really want the higher damage I would lower the health a bit so it needs fewer CC abilities|spell to bring down if the opponent is really desperate to bring it down. So if you want the damage to stay, I would make it 5|3 normal and 7|2 upped. Even with 3 health it would last longer than the Immortal against a spell effect shield, even if it would deal its damage every other turn. The important thing is that it will be alive three turns longer, giving the owner twice as long to counter the shield, if possible. With 2 health, it still lasts 1 turn longer than an immortal against a shield.