I think novaxes are relatively fine as they are, Maybe a slight balance change, but you have to remember a novax is 36k mass iirc, 2 of them costing about the same price of a t3 arty +/- 2k mass, but the damage output is nowhere near t3 arty, yeh arty doesn't have the precision of a novax, but a novax doesn't have the damage of an arty, it takes 2-3 artys of novaxes to be able to penetrate anything heavily shielded, and at that point you could have a Game Ender up.
Yes maybe there could be a way to shoot down novaxes, but the day that happens i guarantee the novax will essentially be the next mercy, and if your opponent has time to build a novax and not get punished that is you not doing your job.
Maybe something could be done about the damage, but if your nerfing damage you have to give it something else, someone mentioned more aoe, but then would that give it shield spill at that point which would arguably be even better than it is now, or then you have to think "oh shit, they can hit two pgens at the same time" which will kill your grid faster, makes targeting engies much more worth, etc, etc...
Something as simple as a damage nerf or way to shoot it down wouldn't work.