Do you think.. *possible spoilers*

Don't forget that Van Hellsing already defeated Alucard, but instead of killing him, he enslaved him. I think Alucard wanted Anderson to be the one to finish him off, and to do it on his own strength, as a human, not really on any additional divine or demonic power source, which is why he was so opposed of Anderson using the Nail on himself. He saw it as Anderson admitting that he wouldn't be able to defeat him otherwise, and it disappointed him. After all, how many times has Alucard said that it takes a man to defeat a monster? He even gave him the perfect opportunity. The reason why Alucard never seems to die is because he's got millions of lives inside of his his familiars. But when he released his restrictions down to Level 0, he unleashes all those lives as an army that he controls. This brings him to his strongest, but also most vulnerable since the only life he had left inside him was his own. Anderson could very well have beaten him, and almost did had Seras not interfered. Now though, it seems he's truly as close to immortal as someone can get since he has Schrodinger inside him, and as long as Schrodinger remains self aware, he'll exist, therefore keeping Alucard indefinitely alive unless he drops to Level 0 again and expels Schrodinger from his body.