If it's the scene I'm thinking of, yeah it's pretty tough. Even playing it on an actual Mega CD with a Megadrive controller is pretty hard. The only way I could get through was to use my arcade stick with rapid fire on. I just held down the button and moved the stick as fast as I could to the oncoming hoards.
Probably doesn't help much with a keyboard, unless one of the emus allows you to select a rapid fire option...
That took me a bloody age too with a Sidewinder. My Sidewinder seems to have some driver issue as well, it kind of ghosts in a direction for a few seconds every now and then. It's the second one thats done it and bloody Microsoft give you no help at all, surprise surprise.
Are you talking about the insectors? All you have to do is wait for them to land, then wait like a fraction of a second, giving them a chance to settle in. If try to shoot them off the moment they hop out and land they'll hop away somewhere else. You have to give them a chance to settle in their spot before you open fire. Then you'll just need a lot of patience as its a long sequence.
Hee hee...that shooting scene took me like 3 tries (with an actual Genesis pad), then there's the one after it (that's a LOT easier) and the final shooting scene....is when I run out of continues. AAAAAARRRRGGGHHHH!!! I had to turn the game off for awhile, lest I drive a railroad spike through my Sega CD.
Played it yesterday, needed four tries on a original sega cd. you should really use a pad for this. just keep pressing, don't wait for the insectors, press madly until your thumb hurts (mine did). then just move quickly to the insectors. don't wait for the insectors and then shoot, keep on shootin !!!!