If Fox likes to Illusion rly high (Above your utilt range, perhaps aiming for a platform, or is good at cancelling it to get max range) you can always sh/fh dj nair to intercept it. But that is if you predict it, for you can't react to it in time. And leave a nade on the ground so if he does illusion low he may get blown up by it. Speaking of which nades are really good way to punish illusion, as long as you leave nades on the ground theres a chance he'll blow it up with illusion or land where the nade is. He can illusion higher to avoid nades on the ground. But if you are in nade stance the nade will be higher and he will blow it up with illusion. In that case it is best to roll towards where he will land with good timing. If he doesnt blow up the nade, and if you avoided the hitbox with your invincibility frames, you can still punish his landing lag. If the fox catches on and illusions to the ledge, leave nades on the stage (dont bother setting up C4 or claymore if you havent already, waste too much time). Nade stance walk to the ledge, then grab the ledge, he'll either get blown up by the nade or die from trying to go for the ledge.
Utilt and holding jab are a couple of rly good ways to hit fox out of illusion too.
Haha I know I just wrote an incorrigible wall of text but hope you can decipher it. =p