Okay. I have a neighbor who mains just Fox. So here's my strategy vs him, if it helps:
Fox is a very fast character, that is very depending on movement in the air. You should definitely try to go to more ground based levels against Fox ALWAYS, like FD or SV. Just the initial thought when going in against a Fox.
Although we're light, with the proper SDI going directly left or right I've survived Fox's fresh U-Smash up to 105-110%! That's pretty freakin high considering OUR weight and his powerful foot.
As far as projectiles go, I say that Lucario has the upper hand. Fox's gun is very easy to dodge and approach against, as it has zero knockback. Using baby auras at low % to get him to approach is a fantastic method, because if he decides to reflect, it'll disappear before even getting to you, and chances are he probably won't even reflect anyway. What is this, Melee
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He's a speed demon, and will **** us in the air. So that's not a great way to approach. Fox's like to use drill (dair) combos as an approach tactic. Utilts are the answer to that. It's decent range and priority in comparison to the drill will stop em in his tracks. And if you decide to approach, most of the time the simple fair-dair will do the trick to get em up. But...
Here's the thing: DO NOT FOLLOW A FOX OFF OF THE STAGE. A second jump-fair by a Fox is a good added 15%, since the double jump will make you sucked into all 5? (i think it's 5, may be 4 or 6) kicks, PLUS gets you the f*** away from him, which you don't want if you're actually following him off of the stage. Now, if you're the one forced off the stage and are being followed off by a Fox, it can be painful. The thing I find the most useful to get a bit of space is a simple fair and scurrying back on the stage. Trust me babe, you'll probably find yourself without a stock if you follow a Fox off the stage. It's suicide.
Now, one last thing to discuss: Recovery. I personally like our recovery, cuz it gives us some leeway. Usually an opponent thinks "Durr hurr imma grab da ledge since dey can't attack". Fox will usually do the same. So for the first few times, just going onto the stage will work. But when that Fox realizes that ledge hogging won't really work well, that's when you go into s***ing your pants mode. Try to make space as fast as possible, just enough space to give yourself time to grab the ledge and get on the stage. It's your safe haven.
Now, when a Fox recovers... we're in the drivers seat. He has two ways of recovering: Foward and UpB. When a Fox uses FowardB, they're not grabbing the ledge, so don't assume that they will. They're looking to land on the stage. That's when you can go into your fair-dair, or your foward B, or whatever you please. Now, Fox's UpB is fun
. A perfectly timed dair will send a Fox either stage spiked, or the opposite way of the stage. And when a Fox does his UpB, your dair>his UpB. And if you wanna take the ledge hog approach, you can since his UpB is pretty predictable. But he'll get on the stage if he's kinda close to it. He gets decent horizontal air movement.
Summary: 55:45 Lucario. Just... don't go off of the stage and you'll win. A good Fox CAN beat you if you're not careful.