I'm putting this in the newbie section, even though I'm not really a newbie. I've just been away from sylestia for a very long time, and some basic things are now new to me.

In the festival zone, I noticed something called the orb of champions. It seems to let you set the difficulty level of the fest zone. It also said that you get better fest rewards at higher levels.

But if someone was hoping to level their pets' levels & abilities as quickly as possible, would the festival zones be a good place to do it? Is easy mode most efficient? Or harder mode? What helps most for building expertise ability ranks quickly?

Thanks everyone!

I also returned recently and had to re-learn all of this.

You can't increase the rank of abilities in the festival zone, only in the lost grove (and maws of madness during fall fest - idk if any other fest has a feature using LG's exploration system).
The increased rewards on the festival orb is refering to enemy drops, so if you can do a higher tier without it becoming too difficult or time consuming it's a good way to get more festival items.
The festival zone can be decent for exp, but I don't know how the orb of champtions influences that.

The fastest way to level pet ranks is the ancient temple at the end of the grove, so what I did and would recommend is not focusing on grinding ability ranks too much until you get there. For black friday there is an exp boost going on, which will also increase ability expertise in the grove.

One thing to be aware of in the festival zones with the tiers is the zone mini events (for fall it's the diabolical pumpkins event) are scaled a lot tougher than the regular zone enemies. I can run through the zone on the highest tier without difficulty, but usually swap down to tier 3 for the events.

Thanks Silver! That really helped!