@CerusSerenade: Only Schrodinger Alucard could do that.
But, first of all. If you mean materialize in his mind, he can't do shit that way other than annoy him. If you mean in his brain as in a teleportation gib, neither he nor Schrodinger have ever shown the ability to do so and to mention it is pointless.
@Fehafare: Schrodinger did it to appear in Zorin's mind. If Schrodinger could do it to appear in someone's mind he could do it to appear inside their brain, as we can both agree that appearing on someone's abstract mind is far more difficult than a physical place.
Ragna without access to idea engine and the grimoire will tire out after trying to destroy all of Alucard's lives and will fall prey to one of Alucard's bullets/impalement.
@CerusSerenade: Yes he did, but still teleportation gibs are only accepted if the person was shown to do so, otherwise it's just speculation. It's not about the place he wants to appear in as much as the fact what he wants to do by doing that.
Ignoring the fact that Ragna pretty much soulfucks and regains energy by doing it, so tiering out wouldn't be an option, Alucard can't put a dent in him even if he goes all out let alone a single bullet.