It's not a "this setting will work for you" environment. Monsoons are something I have yet to do, but from the elements I've read, you mostly want to try to keep the apeture open, but obviously not blow out the image. You probably really want to aim more for the time of day, sunset/twilight time being the best, and go for storms that produced the multi-stroke type of lightning. You are going to have to just spend lots of time with it.
Are you shooting with a Z1U by chance? Those are pretty good with lightning and are one of the last pro level CCD cameras around (although discountinued).