Could we get more info on what it is exactly that you're trying to grind?
I might not know too much about the best grind teams but I'm pretty sure with the way the battle system is split between LG content and other prior content that more context would help.

(Also, just had to mention, your avatar is very pretty. :) )

Well it will honestly depend on your fighting style and what your going for.

For fest contests lower level levels will be easier since the enemies are weaker than if you use a lvl 70 which will take much longer.

For fest contests I'd recommend using light/fire/fire either lvl 30 or 50 (I prefer lvl 30 personally)

As for an lg grinding team a basic tank/healer/dps team should be good.

It means damage per second, but yes it's the attacking one

Tank:
Earth
Water
Shadow

Healer:
Technically only light but water can be used as a support

Attacker/dps
Air
Fire

Dexterity increases critical hit chance and agility increases avoidance chance

Shadow/water/air hmmm I don't know anyone who's ever tried this before so I'm not entirely sure. I could imagine it would work though? It just might be more difficult due to only having water as a support instead of a full on healer like light.

I could help with arranging proficiency points though if you'd like me to.

Yep, shadow/light/air is a very common team to be seen using.

Also is your party your doing for the fest grinding team lvl 70? Contests are good with lower levels but killing fest bosses is better with higher levels so you can gain the better prizes if that makes sense. But yes light/fire/fire is amazing for boards.

I guess I'll just post what would be good proficiency set up for shadow/light/air then? (Note that this is what I think is good, others may have different opinions)

Shadow:
1health, 2 intelligence, 2 agility

Light:
3 intelligence, 2 dexterity

Air:
3 strength, 2 dexterity