WET

You have to kill enemies with one of the special sword attacks you buy in the upgrade store. Of all of these the easiest for me was the slide slash, because it auto-homes on the enemy -- slide by hitting B, and then hit X to initiate the attack. If you're anywhere near a bad guy, you'll get an instant kill -- sometimes two if they're close together.

The dive slash is much more irritating to do but there's one place where you can rack up a bunch of them. In the rage mode spawn door section with the boxcars (chapter 10, "Shock Therapy"), there are two spawn doors on the upper level where you come in. Enemies will spawn two at a time out of these doors, and if you camp out down the hall from one you can jump over them and slash them both at once as they come out. It'll take a little bit of timing, but you can rinse and repeat until they stop spawning.

Since Shock Therapy is a very short level with a lot of bad guys, it's a good choice to replay in order to build up your kill count.