What fun would this site be if it didn't include some game strategy? If you need some help defeating Walter, why not try out some of this handy advice? Success not guaranteed, since this is my strategy, which is probably lame. :p
These strategies are for Leon and Joachim in normal mode, though I'm pretty sure you can apply Leon's to Pumpkin as well. I never played through as Pumpkin since I found playing as a whip-using character a drag after having fun with Joachim's swordapalooza.
Will update later. :p
Unlike Leon, Joachim doesn't have to break the Ebony Stone before fighting Walter, so be prepared to fight the real battle as soon as you enter the room. Interesting that his swords work as an anti-vampire weapon, huh? No innocent soul required.
Fighting Walter (and Death too) as Joachim is ridiculously easy; I just picked up the game to test this strategy since it's been at least two years since I last played, and I had no problem ripping through both in about three minutes. I recommend the stronger sword stance for higher damage and lots of dashing (hold R1 and push X) to avoid being hit. Dash is especially important for avoiding Walter's mana flare, the spikes of white light that shoot up out of the floor.
Keep your distance when not attacking, as Joachim takes more damage than Leon. When it's time to move in, you'll have to find the distance from Walter that works best for you. You'll want to be far enough that Walter can't get in an unexpected cheap shot, but close enough to unload all the swords in one shot before having to dash out of the way and wait for them to return. Of course, make sure you have the first one fully charged before dashing in for an attack. Once you're done, dash out of there quickly and wait for another opening.
By the way, this dash-smash-dash strategy works very well throughout the game and should be exploited whenever possible. Joachim is fast and cheap. Er. You know what I mean.
When Walter moves back to the throne and prepares to use his Demonic Megido attack, dash back as fast as you can and press up against the door of the room. Alternately, if your magic meter is full enough for a special attack, you can get close to Walter and wait until he's just about to unleash the attack, then use Joachim's attack at the same time. Joachim won't do any damage, but he'll be protected by the mysterious invincibility that comes from using magic. This isn't recommended, though, because you'll want to unload some laser beams on Death.
It's really quite easy. :D