Route 115 (Surf), Route 122, Route 123, Route 125, Shoal Cave are the places where you can grind. You can even surf from Routes 105 to 109 and visit the abundant ship. You can climb up the MT. Pyre (inside) to collect TM Shadow Ball. There is one trainer in Route 103 etc.
I am surprised that your team is so underlevelled. IF your 'mons are so underlevelled, how did you get past Winona (Fortree Gym)? Or did you miss it? Nonetheless, if you specify your team, it would be helpful to answer your question more accurately.