Nicely done!
First of all, for only being your second submission to Newgrounds, this is absolutely superb. I see you quickly becoming a respected artist on the site. Please, keep making movies and keep growing as an artist; you'll go places.
What I liked about the flash was that nothing is really executed incorrectly. The sound is perfect; I'm not really sure how you got the DMC3 sound file, but it sounds just as if I was playing the intro on my television. The animation synches up perfectly, and hell, its DMC! It was a nice idea that was implemented with little flaw.
I suppose the real problem with the flash is that all of its appeal really relies on the sound clip. If you really think about it, nothing really happens in the flash. Its the same background, which isn't really all THAT detailed. Its grass and rain. The only sprites (or detail of any sort) we see are Mario and Luigi, holding identical swords, reinacting the very basics of what happens in the actual DMC introduction.
My suggestion would be, if you're trying to reinact the intro, really reinact it! Nothing really happens in the flash. Perhaps add the scene where the swords are clashing together, and the rain goes flying away from the two (you know what I'm talking about). Make it look like they're on a tower. Make their swords look like the swords used in the game. I suppose I'm asking for variety; that's all.
Overall, this is a great flash, and I enjoyed it. I just feel that, before it becomes excellent, it needs more additions. Good luck, and I look forward to future submissions.