Make acu harder to killGet a shield, t2 on acu, or if he spams extreme amounts of corsair maybe a personal shield on acu. Make sure you don't stand next to t2/t3 pgens or energy storage or massfabs.
Get stuff to kill corsairs fastSpam inties. If you have air control be aggressive and kill his corsairs in his base, or while they are on their way. If you don't have air control make flak and inties, and try to get behind his corsairs in an area where you have flak so he can't kill your inties with his inties. Flak alone usually does not work well because if he spreads his corsairs he can still get many volleys off before they are killed.
Most importantly, scout him so you can react to it If you don't scout and you try to react when his corsairs are already over your head, it's too late (what usually happens to me
). Also if you see that he has few units and fex mex upgrades you should get suspicious: where is he spending his mass? Maybe in 10 corsairs... You can also see one of his air factories going dark on your radar: this means that he went t2 air, and maybe it's time to scout again to see what he is making. Especially when you are winning you can spend some mass and attention on scouting, and luckily for you when people try to snipe you it's usually when you are winning
When you are really winning you can also put a factory on scout spam. Don't do this unless you are ahead or if your eco is very big, because it does cost a lot of mass. You can also put the factory on an attack move route and queue up 1 engi 1 scout. Then at least you may get some of your mass back, and you still have plenty of scouts.
Dodging corsair snipe got harder after the "nerf" that lets them shoot from a shorter distance. Maybe the corsair bullets should be slower so you have the same time to dodge as before the "nerf".