How should I use my enemy skill materia?

[Mirage]

That's "almost", at least relative to Trine which is probably 20-25% weaker.

Aqualung is still much better than Trine, and can still be gotten much earlier than Trine. Trine is actually one of those I use the least. Only when I'm up against a target weak against lightning, or when I don't need the amount of damage Betalung offers.

Enemy skill is most definitely tied to the person's magic power stat, though, as evident by Earrings/circlet increasing damage on these three main AoE nukes significantly.

[Big D]

Pandora's Box is a fun enemy skill, especially since you only ever get one chance to learn it. Miss it that opportunity, and you can never get it again.

[Mercen-X]

Don't use them... apparently the game's more "fun" without them.

[Jiro]

That depends on definitions of fun. Slaughtering enemies is pretty fun imo.

[VeloZer0]

I agree. You have to balance the enjoyment you get out of applying yourself to a challenging experience to the joy you get out of nuking everything in your path.