I assume you probably have the most trouble offstage and when jigglypuff can get inside.
Refrain from using fair onstage, because jigglypuff can get around it fairly easily. Nair and sometimes retreating bair is all you need, really, for spacing. Not even sure if jigglypuff can get past bair walls that easily. Jigglypuff will try to break your spacing with her aerial speed and go for grabs. Even though our tilts can easily beat jigglypuff's aerials, retreating bair is often easier because it's less punishable. it's best to use tilts out of jab or shield.
So pretty much spam nair and bair, ocassional jab, and abuse your shield because jiggs isn't going to do anything other than aerials and grabs (and she'll do it from landing or shield grab, not a dash grab)
In case you have trouble getting gimped,
Jigglypuff can wreck Ike offstage given the right positioning and lack of knowledge, but if you get knocked offstage, it's best to fast fall and recover low, because jiggs can't follow you fast enough. If you're for some reason horizontal to jigglypuff offstage and you're not that high, you'll probably get gimped. Jigglypuff. If you don't use aether by throwing the sword forward and moving back, jigglypuff can actually fair the aether pretty easily, and can also hit Ike out of even reverse aether before he rises due to her air speed, but it's a risk for jiggs to go that low, because if she gets hit by aether, she's pretty much dead at low-mid %.