I must have fought this guy 15 times when I first got the item from headmaster Cid. I still haven't beaten him, help!
In FF8, how do you defeat Diablos when you first get him
final-fantasy-8
Related Solutions
It raises stats, though Junctions raise stats far more effectively for the most part. The main purpose of leveling is to raise enemy's tiers--higher tiers (the max tier is actually very low, around level 40 I think?) drop better items. Higher tier enemies also use more dangerous spells or attack patterns however.
For instance, Elnoyle will drop Moon Stones at low tiers, but Energy Crystals (useful for gaining Ultima) at a high tier.
Also, certain encounters have fixed levels. But generally speaking, leveling will actually make enemies harder, though they will drop better item rewards. Much progress in FF8, unlike most other FF games, is related to items directly or indirectly, so they shouldn't be ignored. By fighting higher tier enemies you can more easily synthesize high level magics, weapons or GF ability items.
Grinding in FF8 is rarely direct, aside from AP. (It's also rarely necessary) Grinding to level 100 may seem pointless, but grinding Energy Crystals will give you plenty of Ultima which is both a very useful spell, and the best stat raising junction for several stats.
You can also grind if you WANT to make the game harder. Personally the first thing I do when possible is head straight to the Island Closest to Hell and get to level 100 (which due to FF8's leveling system and the level 100 enemies, does not take long).
Ouch! Your fears are justified according to this Steam poster:
You need to have all the cards together at one point in order to get the achievement. At least 1 of each. Getting the gold star for getting them all won't get you the achievement. (Need to be holding 1 of every card to get it.) You can get back any rare cards you missed or modded in Disc 4.
(Name: Senrei, link: http://steamcommunity.com/app/39150/discussions/0/648814395976705191/)
So looks like that second playthrough will be needed.
Best Answer
As I recall, Diablos is pretty challenging when you first get him if you haven't been doing any grinding or excessive magic-drawing early on.
The key to the fight is the spells that you can draw from him. He always carries Demi, and will have either Cure, Cura or Curaga depending on his (i.e. your) level. Draw these and use them to your advantage.
The Demi spell instantly removes 25% of the target's current HP. This means you want to draw it and cast it as your first couple moves. The more HP he has, the harder it hits. In this way, you can drop him to half health in two shots.
From there, beat him down with standard attacks and magic. As a flying monster, he is weak to Wind. Make liberal use of the Cure-family spells you can draw from him.
His Gravija spell hits extremely hard, but it works the same as Demi, it just takes a much higher percentage of your health. If he puts you into position for a Limit Break, you can also do a lot of damage that way. (Remember, you can hit the Circle button to retry for a Limit Break if you're at low HP).
Finally, don't be afraid to use any healing items you have. Items are plentiful in FFVIII, and you'll be able to easily resupply later on.