Diablos' damage is based on his level and the max HP of the opponent. He
uses a demi attack which does 10% of the apponent's HP per 10 Diablos
levels with a max of 9999 HP.
To put it plainly, at lvls 10-19, against an opponent with 10000 HP, diablos
will do 10% or 1000 damage. At levels 20-29, he will do 20% or 2000.
At lvl 100 he will do 100% or, since the opponent has more than 9999,
he will only do 9999. As I said, it's based on max HP, so if a monster has
10000 max HP, he will do 9999 and then on the next turn, 9999 again.
While most bosses are immune to him, he is by far the best GF when it
comes to killing normal enemies late in the game. While many of the GFs
are great in the beginning but much less later, Diablos stays strong
throughout the entire game. The only GF who will do more damage than
Diablos is Eden, who has no HP cap, but you need to level her up and train
her which can be time consuming seeing as you get her near the end of
the game (either from Ultima Weapon or even closer to the end of the
game from Tiamat) as compared to Diablos who you should have had
since the moment Cid gave you the Magic Lamp. If you didn't rub it
right away, then poo for you, Diablos gets MUCH harder when his levels
go up. ;]
So to sum up: Diablos is my favorite. ^_^
.opt