I'm on the low end of the MZ level spectrum, and I'm looking to level up my pets more before trying to fight the Behemoth again. The Lost Grove seems to gain experience much slower than the previous regions, although it's the only place where you can gain expertise in attacks.
My party pets are Thorne, Plague Bearer, and Tianavide. Where would be the best place to level them up?

Dungeon of the Forsaken in Umbra Forest seems to be a good spot.

I've been using the Dungeon to get an experience boost each day... Otherwise, I find the area towards the Captains pretty good given each battle often involves more than one opponent - you don't get a bonus for battling more than one (only the standard exoerience x the number of opponents) but I find it faster. I also try to battle the elites / named as often as possible.