This is the only diffiicult Jenova in the game and she can be a bitch at times. The only advice I can give is use summons and the best magic attacks you have and keep healing regulary and also use your limit breaks when you get them. If you are still having trouble after trying this walk around the ship for some more EXP and try again and also off the marines you can steal a decent piece of armour called the Shinra Beta you could equip these and see if that helps hope you beat her soon!!!