He's durable enough to grab Walter's wires with his bare hands with no injury at all, is strong enough to kick Alucard in half and force him to retreat, is quicker and more versatile with his rapid shapeshifting, and most importantly Walter has no silver.
The Captain is better than Walter in every way and is only vulnerable to something Walter does not have on his person. The Captain is easily strong and fast enough to destroy Walter's head, killing him instantly.
The Captain is the most powerful supernatural character in Millennium's service, and in the top three which includes Alexander Anderson and of course Alucard. Personally I'd say The Captain is more powerful than base Anderson, though not as strong as Monster of God Anderson.
Yeah, Nemebro outlined how Cap would be a horrible matchup for Walter.
I can understand Vamp Walt beating Base cap, but Cap is faster in his werewolf form meaning he can better react to his wires. Cap likely gets a durability boost which means it takes more time for Walt's wires to shred into Cap's flesh, assuming they can cut into him at all. This gives Cap enough time to shift into mist, phase through all of the wires, reform back completely healed, and then kick Walter.
Walter doesn't have the same durability as the other Hellsing top tiers, so he's going to get one-shotted.