Haven't beaten them yet, but soon will. I intend to detonate their home systems in a few turns=-)
I was lucky and started on the opposite side of the map. All the AIs have gone MEE on the monsters and have formed a massive alliance against them.
I've found that the monster PD can be fairly easily overwhelmed using external mount missiles. Once you get to about cap missile IV you can out-range them as well.
Oh, and watch out for monster intel, it gets really irritating...
I've yet to take on a worm, but I've seen the design and they look practically untouchable... thousands of shield points, thousands of armour points, regeneration ability... maybe with a few massive mount WMGs, but I think I'll just vapourise their homeworlds and watch them starve when their maintenence is cut off
