Put the minefields around your planets if your opponent has warp-point generators. He has to come after your colonies sometime, and like others have said, mines ignore shields. As for engine-destroying weapons, build a couple (large) ships with lots of the engine-destroying missiles. Overwhelm the PDC (and maybe add some fighters, just for flavor, and to keep more PDC busy). And, of course, load up on armor (or phased shields) so the Phased-Polaron Beams are less scary. I'm up against an AI that uses PPB's and got used to staying out of range (I only had standard shields on the ships in that region), and in one battle (tactical, not strategic) thought I was in trouble when I failed to stay out of range. Then, as I watched the enemy fire, I realized I had finally gotten ships with phased shields deployed... what a relief...
