Alternate method: Load up the first stage in chapter 3 which is Kowloon I believe. I used a fully upgraded Gunner (Dual Kimbers) and from the very start of chapter run to where enemies start to spawn and walk through the level guns ablazin'. Continue walking forward and spawning enemies while firing the entire time. Should be able to get your combo up to 50 very quickly. I would assume you could use any character (fully upgraded). Very easy to get even on hardcore via this method.