I need help in Final Fantasy VIII?

I have Shiva level 2 junctioned to Rinoa level 11, Quezacotl level 12 junctioned to Squall level 14 and Ifrit level 10 junctioned to Irvine level 13. I have 20 potions, and 5 tents, but I can't use tents in battle. I igned Elemental Strength-J to Ifrit and no magic to Quezacotl. I'm at the part in disc 1 where you climb out of the sewers as Quistis and pull the lever to trap Edea. I can beat Seifer easily but when I get to Edea, she pulls 400s on me and I use potions but they don't work because she takes away the amount of hp I restored plus more instantly after I use them. I keep using Ifrit's Hell Fire and Quezacotl's Thunder Storm, but they only do about 600 dmg each. I REALLY need help.