Sandy is like two megajoules. She's only twice as strong as frank that's not a one shot territory in the slightest, and Frank's sniper rifles can take down people at least somewhat comparable to him (the zombies) in one shot, so he's going to be causing a lot of damage, especially if he shoots her anywhere vital because thats just how organs and a damn gun works.
Sandy's jetpack isn't going to be invisible, is my point. That's the part that frank is going to shoot at, and a sniper rifle meant to shoot from very, very far away is gone be very helpful. Hell frank also has all the time in the world to just set up motion tracking turrets or any sort of trap to just shoot sandy out of the sky.
This is assuming that sandy starts with either invisibility or flight in the first place, which she hasn't been shown to, with invisibility coming from a one off episode and flight being the only one she might do to get close. Show me that sandy wouldn't immediately go for flight or karate and I'd accept it but I don't think she uses many of her inventions in an actual offensive way, unlike frank who does that shit all the time.
Also if size isn't equalized I doubt that Sandy's jetpack can go too high up considering her size. Assuming it can reach even past the treetops is a bit absurd considering she's smaller than a damn pineapple in-series. Assuming anything of hers can get that kind of height on it is either NLF or wanking (don't care enough to tell ya which).
If sandy is small, her size kills her in the end because she's just way too small and her equipment is far too noticeable and frank has far too much range with a scope that is meant to hit targets from far away, and her range is crap. Frank is arguably lore experienced in fighting, and has the much more versatile and ranged arsenal to go with it and a bigger killer instinct as well.
So frank has better stamina, range, versatility, experience, and instinct. And I say experience, by the way, because sandy hasn't fought in severe zombie outbreaks and killed several psychopaths and military commandos or mercenary groups with actual guns that far outrange her.