User blog comment:Omega natsu2/Episode 179: Gray vs. Rufus, Review./@comment-288216-20140426233755/@comment-7418318-20140427094654

@Anon: You're forgetting something. A spell doesn't have a set strength that's independant of the caster. It differs from individual to individual.

Just look at Macao and Romeo. Even though they use the same spells, Macao's purple flare still has a lot more punch to it. Why? Because he's more experienced and has a bigger magic-power pool to work with.

We don't really know how much pure power Rufus has available. If he has comparetively little power available he can invent or learn powerful spells as much as he likes, but still won't be able to do more than scratch their true potential.

Furthermore he used all those spells for the very first time. Maybe even he needs a few runs of trial and error before he perfects one of his invented spells, but he was so sure that Gray could never defeat him, that he continued on experimenting instead of using spells he already mastered perfectly.

"Defeated by their own arrogance" is a very common thing for antagonists in Fairy Tail after all.