Alucard can summon Rip Van Winkle (since he ate her) whose musket "punishes all without distinction" (including missiles, helicopters, multiple targets with one shot, one target multiple times with the same shot, etc), or any of his other familiars (the people/animals/things he's eaten over the years). I don't know Xadhoom that well, but judging from what I've found on various wikis, her nuclear power would likely be her downfall, since they say that if she overexerts herself she'll explode with the force of a small nova. Alucard could easily overwhelm her if he's released of his restraints, since he can summon everything he's consumed (including some fake vampires, a few popes, and many, many more) and send them at her. Or he'll just go right at her, using the lives he absorbed already to survive what she throws at him until he can kill her. I know she doesn't need food, since she's nuclear-powered now, but she's still in need of things like a throat, a head, a heart, correct?
I'm more curious about what happens if he tries to drain her.
Rip's bullets would fail for the same reason Alucard's would; her incredible body heat.
regarding the anatomy things: her physical body's a mixture of pure matter and energy. but missing a head would probably still be dangerous, if not fatal
and because of that, i'll imagine she doesn't have blood..... i may be wrong.
something that may or may not be relevant is the fact that she has on occasion turned parts of her body into flame and back...