Sephiroth vs Alucard

Galactus devours Alucard's existence and erases him from every level of existence. Even Silver Surfer (or any Herald really) would bitchslap Alucard

As for how Sephiroth kills Alucard, well unless shite like Death works, then hard to say if he could defeat Post Schrodinger Alucard

There's always BFR though

I mean, I recall in Hellsing Ultimate where the Saint guy (I forgot his name) used his demonic power to crush Alucard's limb regen.