We live under mostly rock so we decided to make a raised flower bed (about 60" x 20" and maybe 6-8" high) to minimize watering and enjoy some local vegetation. We currently have bermuda grass in the area and I'm wondering what the most effective and easiest way to deal with that is.

I've been reading about different ways to get rid of grass and my favorite thus far is the "lasagna" or smothering method, seems to be the least physically strenuous way. I've also read that bermuda grass is VERY hard to kill. Is smothering likely to take care of it or will I be fighting a loosing battle later on? What if I use RoundUp and then smother it? What should I do??? Thanks!