Be aware, though, that precursor squadrons often operate near a robotic listening post that contains a large ammo dump.
My suggestion would be a force of cheap, disposable FACs (or small ships if you haven't researched FAC engines), each with a number of box launchers (or the most reduced-size launcher you have researched). Design a missile that goes as far as the precursor missiles can reach and has enough spare fire-control range to account for their ECM. Give the missiles small active sensors in case the launch platform is destroyed before they arrive. Make the missiles small, size 3-4 or so, but with at least Damage 4. Fire everything you have as soon as you can, all at once. Aim for at least 5 missiles per 1000t of precursors, and split what you have among their fleet. Blow your entire load in one volley, so to speak.
Your doctrine for this engagement is "At least they'll die too!"
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Edit re below: Good point. I hadn't thought of that. Spoilerfied.