Jiggs is gay, bair is really your only reliable option, but in the battle vs Jiggs you really can never let her get too comfortable. If you aren't pressuring her enough with bairs and stomps and she has time to set up the wall, you're in for a ****ty stock.
Try to keep her at bay, your opportunities to attack are not frequent vs Jiggs, and running in is scary b/c of utilt>rest, duck under grab>rest, and getting grabbed uthrow>uair>rest. Once you do get a hit and Jiggs is on the defensive, keep attacking, put pressure on and make sure you capitalize on her mistakes.
The grabs you do get should be followed up by dthrows, and if you don't think you can knee her just go for a uair and try to stay under her.
But answering your question, if she has already set up the wall and is doing that crap, if you're in the middle of the stage you really don't have much to worry about, but if you're at the edge of the stage you really want to try to get around/behind her. This is difficult but work your sheild, uair if she goes high enough, dj over her if she stays low, and otherwise just turn around and try to counter her moves w/ bairs.