Phoenix - High School DXD [Marvel]

The idea is for the character to attain the White Phoenix of The Crown. Of course, the character won't have instant control of its powers, but with a span of 20 years to reach maximum power. Let's be honest. Without Issei gathering his harem to gain power through them, the DXD world will be destroyed. So, the character must at least be OP to handle the EXE. ['There won't be a Phoenix. Because the character himself will be the Phoenix.'] Yes, he will be OP. No, Sirzechs is completely incapable of killing him. How do you kill a cosmic entity?