found this while tryin to kill vamp on extreme....
you don't need to use any items at all. see that hand railing in the back? you can flip over it and hang. simply hang and vamp will repeatedly stop what he is doing and walk over to you to slash you. as soon as you see the slashing animation begin, press action button to get up and rapidly tap punch button to score some hits. anticipate his attack a second early to score even more hits. it works because when raiden is flipping/getting up from hanging he is temporarily invincible.
i timed it, it takes like only 8 minutes. amazingly, it worked through the entire battle. try it. --- I've been wandering for more than a century and now my journey comes to a close. What a splendid way... to end it all. -The End, MGS3
The real cheap method is on the Extreme Speed Playthrough. Hanging off of the platform in the center, then killing Vamp in one big punch chain. --- "At 99:59 Megaman evolves into Raichu and you get Elec Pokemon Style level 100. I think." Best thing I've ever said.