I played against a few (good) Jiggs online and was surprised at how difficult they were. As the guy above me said, arrows are practically useless - they fly right over her head when she is on the ground or else she can just easily jump over them, just like she easily jumps over the boomerang. I found bombs to be the "best" projectile against her, but even then it was difficult to hit because she's usually in the air, and her abilities allow for very sporadic movement, so it is hard to predict where to throw in order to land a hit.
Ultimately, what I found to work the best was to not even use projectiles, but rather just wait patiently while she floats around in the air and then try to punish when she finally has to land. It can be very difficult to keep track of how many hops she has left, but if you can, it gives you a great opportunity to go in for a big hit like a dash attack when you know she HAS to come down. Most of my failures were getting greedy and trying to go for the kill too early. It is very easy for her to float above the range of your sword and then swoop in immediately after you swing and miss to punish, so you've just got to wait it out.
The worst is if you get caught in the air and she gets a chance to charge that spin-move on the other side of the stage, where you can't possibly get close enough to attack from the air. Link's landing lag makes us sitting ducks, as soon as we land Jiggs launches and hits us for a ton of damage, often KOs. I found one way of dealing with this was to have a bomb prepared and then when you get close enough, throw it straight down to hit yourself and launch yourself back up in the air... if you manage to do this, sometimes they will anticipate your landing and go for the kill, but then miss because you went back up in the air. I got one guy to kill himself by flying off the board after he unexpectedly missed me. lol But then he was more patient after that and made sure I actually hit the ground every other time. It's even harder because Jiggs can hold that move forever, and all they have to do is RELEASE the button, so their reaction time is just so fast... but even if they don't fall for the "bomb yourself" trick, you can potentially use that to try to get closer to them, hopefully in range to disrupt their charge with some kind of attack.