I just got a full maxed restricted team of Fabled species, and because I love them so much, I would like to make them my best team, for the Board and the Storyline, if that's possible. I've heard that Level 30 is the best type for Board, so that's that.
Now I just need to know the Proficiencies. What is the best combination of Elements for a good team? Which boosts do I attune the Proficiencies to, in order to get the best result?

Board teams are typically Light/Fire/Fire, or Fire/Fire/Fire
I typically go all-int on mine and let their equips cover the dex (when you're ending a battle in 2-3 turns on average you don't need to worry so much about evasion, especially at lv30 where it's easy for the light tank to just heal any of the meager damage enemies manage to inflict
I haven't used a 3fire lv30 team, only a lv50 team that had Ancient Relics to cover their healing needs

But as to non-board
well.
there isn't really a "best" team?
To profs, Light healers and Air+fire attackers at lv75 are typically 3offense/3dex, but tanks are generally different person to person

Although when fully ranked I believe water/air/air is gaining popularity for doing the Temple?
I used to use light/air/fire for Dia and Kowlla but with the raised level cap I can now easily beat them with my main team (earth/light/air non-max, who have additionally carried me through the Woerm fight and have beaten all the temple bosses)

Some people swear by shadow tanks, others by water, others by earth. It's preference
Light is a common fixture but now that water can hit lv75, it's become the buffer it was born to be