You did that using FACs and multi stage missiles, right? How exactly do you make a multi stage missile? Design the payload first and the bus second?
The missile design window has a section for adding a second stage, it's just below the engine area. You need to design the payload first so it will show up on the design window, just like engines.
You select how many of the payload the missile will carry, as well as separation distance. Separation distance denotes how far from the target the bus will release the payload. Generally you have the payload being short ranged versions of your standard ASMs, the bus can be barely faster than the targets speed if you want, but generally you'll want them to be not too slow otherwise they'll waste fuel and take forever to intercept.
Edit: I believe he used the following strategy for massed single missile salvos.
Start with a FAC that has excellent speed, add a single size 1 launcher and a magazine. Design a missile that's only as fast as the FAC but has good range.
Send the FAC on a direct intercept to the target firing missiles as the launcher reloads . As the ship travels as fast as the missiles, every one launched will be grouped up with the launcher and hit in the same increment, as long as you mass lots of these ships you'll overwhelm enemy point defence.
In fact you can use multiple launchers per ship if you don't mind the micromanagement involved with ripple firing one launcher per fire control per 5 second increment.
