Big Bang Build for my F/P

Poison Breath is not worth it. You'll just use Flame Arrow till you get Explosion and then Paralyze.
Poison is a terrible thing post Big Bang, not worth to max things for it, since Big Bang demands immediate training, the kind of poison mist, change channels and keep coming back is obsolete after that, when you're just hella powerful with explosion with a marvelous range.
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For Elemental Composition, let me tell you my opinion. For Ice mages, at all, Elemental Composition is definitelly not worth it. A reason? When you have a 140%x5 attack, why would you want a 480% damage single target skill? Not only that, but on the few situations you'll need a single target skill, Thunder Spear would be hella better than that. I'm saying this because of the experience i had with seeing some IL mages on KMS.
But FP mages seem different. They don't have two single target skills at third job, but two mobbing skills. Go for Composition, you'll need an 1x1 attack.

Remember I'm not leveling through the jobs anymore, I'm already 4th job. Elemental composition would only be for a use every 10s for its DoT. Same for poison mist (but that one has been proven to be good for DPS already).