R1: Possibly Anderson, so long as he has knowledge that Alucard's hands >>> his guns. He should not have been one-shotted like that otherwise. I have a feeling Helena's Nail once struck and incinerating Alucard would prevent him from using his lives/familiars for regeneration/respawning purposes, thus killing him even if they're all inside of him. Plus he can concentrate his vines all on Alucard instead of the millions of familiars all around London city.
R2: Alucard, especially if he employs some strategy like useing his army to blindside Anderson and then blitz him while he concentrates his thorns on taking out the familiars.
Seras is simply not strong enough. Captain isn't hard to kill as both of his opponents are top tiers armed with ample amounts of silver/specialized weaponry.
Walter has questionable durability and a grave instability in his form.