Complete every mission with fleet customization with Missiles Only. Good luck. In other words, you are not allowed to have railguns, coilguns, lasers, blast launchers, or conventional guns. The only weapon you are allowed to have on your ships, drones, and missiles are regular Carrier modules. However, all payloads are allowed for your spacecraft, so nukes and flak are allowed. There is no limit to the exit velocity of those said carrier modules.
I don't really see this as an interesting challenge - nukes melt missile and drone fleets just fine, while both nukes and flaks can take out stock ships with relative ease. You basically hang a bunch of launchers and a lot of ammo cans off a propulsion stack and call it a day (note that you can launch flares from conventional carrier module).
Now, if we had to do without nukes (meaning actually hitting drones/missiles with own missiles - which is something I have never been able to do, or being able to outlast them), or try to hammer a mass driver out of conventional carrier module, or at least fight at melee range, it could get interesting.
More like KKV spam. They can be small and light, especially since you might ditch armor (for missions with no lasers).
The problem with KKVs is against any fleet with a decent amount of decoy launchers (such as the vesta fleet) and thanks to the homing logic you will need a seriously massive lag-inducingly huge salvo of missiles in order for any of them to make it to a real target. Even with the irradiance cutoff at 0 the homing will distribute more missiles to larger signatures (which are usually flares) and those that go for a real target initially will often change their minds and go for a flare anyway, or just waste all their dela-V going back and forth trying to decide which signature they're going to go for. Of course you could always cheese the AI by turning your missile engines off and waiting for the enemy to stop launching flares but that takes a while and feels a bit like cheating, and if the enemy ships have spread out a lot by then your missiles might fail anyway. A completely manual missile might be worth a try, I haven't had a proper go at making a missile which would work with manual guidance yet.
Even with the irradiance cutoff at 0 the homing will distribute more missiles to larger signatures (which are usually flares) and those that go for a real target initially will often change their minds and go for a flare anyway, or just waste all their dela-V going back and forth trying to decide which signature they're going to go for.
This again shows how badly we need an update. I do not think I'll ever be able to produce a patch for this erroneous inappropriate behavior of missiles remotes by disassembling and poking around. :(