At level 30 in Atlantica, there is a side quest to kill the Night Fox. As the mobs tend to come in sets of 2, that its good if you are in a party. Its great experience and chances are you will need to grind a bit before you can activate the Insect Nest quest chain. Most of the mobs in the Insect Nest also comes in a group.

Remember that, the flying enemies and the Worker Ants are immune to cannon fire so you have to rely on skills(Atlantica Gold). At this point level 35 equipment starts to drop in the form of Insect Equipment Box. You'd better upgrade level 25 armor using Enchant Stone, because level 35 equipment is expensive and Enchant Stone is even more expensive and not worth investing into at this point.

A +4 level(Atlantica Gold) 25 set will last you until level 50. Try to get +3 Asura weapons if you can as well. The best spot to train is at the Worker Ants in the beginning. The Tarantulas and Hell Spiders are not bad if you can kill Hell Spiders in 1 turn. If not they will Deadly Shot you into oblivion so if you attack them make sure you can kill them. You need to stay until level 40 when you can continue to the next dungeon.