Akatsuki has several strong weapons here.
Kisame's mobile lake is a big threat to them (water weakness, and none of their firepower could get through). Itachi's and Sasuke's amaterasu is of course quite deadly and something that's useful even against Anderson as it'll keep burning endlessly even as he regenerates. And of course, Itachi's Susano'o bypasses all matters of regen by simply sealing away the opponent, and Madara can do similarly with warping.
Sasori disarms all weapons with his Kazekage puppet, no wires for Walter, guns for Seras, or blades for Anderson.
Sasuke's Kirin and Deidara's bombs are a good way to kill any foe, or multiple foes, at least once.
And then there's Pain.
Effectively 13 to 4 odds (or more, when you throw in summons) means the Hellsing people are just outnumbered and outgunned. The only thing they have going for them is how hard Alucard and Anderson are to kill, and even that has limits.
Close numbers would be a good fight, but this is just stompy.