Hello. Something I found last time when I played it on PS4, it still works but seems like they increased the odds of meeting Zapt / Fritt / Bablizz, so it'll be quite tough.
Make sure you have Copper Gnome and Moogle or Floating Eyes in your stacks and play normally until you cleared Saronia Harbor.
Train your Copper Gnome to level 10 then transfigure it to Mithril Giant and learn the ability Smash. The next thing you want to do is learn Flutter from either Moogle or Floating Eyes.
Once you have gotten both that abilities, go to Nether Nebula, head on the map where you encountered Shiva/Ifrit/Ramuh, go bottom right and Smash the boulder, then Flutter across.
Start training there, don't be stingy and just use your potions. Everytime you clear one wave, you should be getting one level IF you're about level 15-16. Go back to the save crystal to regenerate then repeat. Keep repeating until you reach level 20+, or whatever number that you feel comfortable.