Snake's ftilt has equal priority to pit's forward B and ftilt combo. If you're fighting a Pit who like to spam Pit's forward B you can easily punish it by spamming Snake's ftilt combo. If the Pit you are fighting is spamming Pit's ftilt combo you will probably need to try a grenade defense. Pit gets the second hit of his ftilt out before Snake can throw out another knee and you'll get punished. I want to try using Snake's knee to cancel the first hit of Pit's ftilt and following it up with a neutral A combo, a dodge, or a shield to see if there's other options there but I haven't gotten the chance yet. It's usually a bad idea to try to roll through Pit's forward B so try to avoid doing that.
If you try to Cypher over Pit and drop C4, be aware that he can shoot an arrow straight up and knock you off the top of the screen, even at low percentages. When Pit's got you in the air you can pull a grenade and let him blow you both up when he hits you, drop C4 on him, try to get in position to hit him with Snake's back air on your way down, or try to land where you have C4 planted so he'll get blown up for trying to get in position to upsmash you. Air dodging through Pit's upsmash doesn't work too well since Pit's upsmash has two hits to it. You'll dodge the first but probably get hit by the second.
If the Pit you are fighting is spamming arrows, crawl and dodge your way toward him then punish with a down tilt. Snake's down tilt has good range.
As always with Snake, keep your nades, C4, and prox mine in play to reduce your opponent's options. A good Pit will probably hit your prox mine with an arrow, so the prox mine is less important but can still be useful to keep him away from a spot where he likes to spam arrows at you. Edge guard against Pit with the nikita and/or high mortar shots. Cheese him with Snake's grab.