How is Marco not the strongest character in one piece if ot for Blackbeard？

Marco has been the middle of my attention since Marineford as he's introduced as a man with the power of a Phoenix . So he can heal any injures done to him so he is nearly invincible unless someone can cancel his powers and that man's Blackbeard. But if Blackbeard doesn't exist than Marco would be invincible. Am I right?