I'm no expert with this, but I'll try to help as much as I can. XP
So, before you even start rondoing it, put a single layer of resin over the outside. A thin coat, that covers everything. This stiffens the cardstock. Then you can rondo.
For doing the rondo step, first take the fiberglass resin and bondo. Then do a 50/50 mix of both. Take the fiberglass hardener and put as much as you think you need in there. Mix mix mix, then pour into the pistol. Slush cast it about for a bit, but try to get everything inside the pistol. Add a second layer if you think you need it. Pour in the bottom plasma injector, and slush cast that first, then try to get the center beam. Once you do that do the top part, but be pretty careful, as if you screw it up you'll have to do it over again.