Unlike the other Final Fantasy games, leveling up almost always does more harm than good. Reason being, enemies (including bosses) level up with you and their stats increase at a much faster rate. Ideally, you'd want to draw and/or refine lots of high-level or related spells for each of your stats (e.g., water for strength, protect for vitality, shell for spirit, haste for speed, etc.).

Your time is better spent drawing magic and figuring out the best way to junction it. Once you have enough magic to junction, start with your strongest character, go through each of their stats one by one and attach whatever magic makes them strongest. If you can junction to a characters elemental attack and defense, make sure you do that too. Properly junctioning characters is very tedious, but then again so it leveling up.

Island closest to hell kill the stuff there save after every 2 battles or so

Get diablo junction enc-none run around island closest to hell for the best magic if you want to get stronger leveling dont matter much

Remember you can use GF abilities to make potions and other stuff thats how I did it I cant remember how exactly but I know I was out of almost everything then I used abilities to live a bit longer trained and got more potions after battles then killing Cerberus hes a challenge if your unprepared but I knew what i was going into so it was all good

Just Keep Trying

I think there are monsters in the halls of Galbadia that you can draw quake from. That is a good magic to junction to str. Then junction Life or something to HP, and Protect to Vit and you you should be able to withstand Cerberus's attacks.

You are in the most perfect place for my level up trick to happen. Go to the ice rink, you won't regret it.

A great place I found to level up was in Galbadia garden. On the Ice hockey court you'll come across monsters in Ice hockey uniforms, every time they are beaten, no matter what your level is, they will give you over 1000 experience if there are 4 of them and just under 1000 if there is only 3 of them. I leveled my Squall, Rinoa and Zel to level 100 here pretty quick.

Galbadia Garden is the same place you fight the Cerberus, but it can't happen before you battle against Galbadia, only when you invade their garden.