Zelda's a slow character so you can spam lasers on her to rank up damage and not having to worry about being punished because you stayed in the laser position for too long (SH double laser works as well). You don't have to worry much on her forward b because you can easily shield/reflect it (down b does not reflect it back at zelda, it just acts as a shield). This makes it so you don't have to forcefully approach zelda and it gives you the freedom of staying on the defensive. In this situation of ranged battling, Fox overpowers Zelda. So if you keep shooting lasers, it's likely that zelda will be coming for you instead of you coming for her, making it easier on your side of the fight.
Another helpful thing to know about Zelda is that she does not do really well in the air, so if you want to keep pressure on the player, try to keep zelda afloat constantly which should'nt be hard for fox since most of his attacks lead to tossing the opponent into the air.
As for approaching zelda and depending on how good the player is, you can either dAir > uSmash at high percentage or if she spams smashes, you can charge at her > block > grab.
How to K.O. Zelda - uSmash @ 95%.
-Gamble