Before we can help we could we have your character stats.

Make sure you have 100 magic junctioned. Thats the most you can collect on one magic for one character. 100 magic will boost your stats the most according to how your level, your stats, and how powerful the magic is. In otherwords, junctioning 100 magic makes the differenve happen, that and how powerful the magic is.

So tip #1 is junction 100 magic!

Make sure you junction the right magic. Cure is a HP junction, it doesnt really work for strength. Make sense? Attack magic is good for strength, healing magic for HP, protecting magic for defense, etc. Experiment and see which one raises your stats the most.

So tip #2 is junction the RIGHT magic!

GFs work like this, the more a character uses a GF, the higher that character's compatibility will get. And the game is very beatable without SUMMONING GFs, but it will be extremely difficult not junctioning them. So if you have good junctions you will almost never use GFs. The trick with GFs is learning their good abilities first and then applying them to the character. HP-J and STR-J ahould be the first ones learned and make sure every person has magic junctioned to them, since HP and STR are the most important stats.

I declare tip #3 to be: Junction the GFs and make sure they learn their best moves quickly.

If you are having trouble with Diablos, try using his own magic on him. Diablos has Demi ( a powerful magic in the first disc ). Draw and cast it back on him, this will reduce his HP by 1/4 every time! What demi does is reduce a targets HP by 1/4 everytime. Have one person cast demi on him, one person healing ( i believe Diablos has cure also, so draw it from him and cast it on your party ), and one person attacking with normal attacks/limit breaks. If you still have trouble, go level up your charcters a couple of times, ot better yet, look for stronger magic to draw.

Tip #4 is hereby declared! Thou shalt level up thy characters and draw strong magic from thine enemies.

These tips are in no particular order, and I know that was a big read, but I hope it helped. Enjoy playing the best RPG ever made!