Theres really 3 options, option one is to just keep throwing ships at them untill they run out of missiles, this obviously isn't the best option, but it's the strategy usually employed by NPRs, option 2 is to make sure your own missiles significantly outrange their defences so your armada can bombard from range with impunity, you still need a huge quantity of missiles to get through the AMM spam though, and option 3 which is rather an exploit is to have a small handful of ships fire many small salvos in an attempt to get the NPR to waste all their AMM in interception, ideally I think you would put a small active sensor on your missiles so your ships could zoom into range, fire, then zip out again to reload or dodge any incoming defensive ASMs. The active sensor would allow the firing ship to leave fire control range without the missiles fired self destructing.
Ideally you want the enemy to waste his AMMs by firing them at your AMMS, if it takes several of his to shoot down one of yours then you have succeeded.