I suggest keeping roughly the same army as last round. Don't add the dragon riders because they'll become more powerful of they're partnered. When you enter you'll find the lower world heroes on the run. They'll be chased by Mugen's army again. Move all of them west trying to keep them in the same space. Gon can only move three spaces so he'll slow you down. Once they're in safety start moving all of your units down. If you make sure no units can get through to your HQ then you can send all of them to fight. Releasing or executing everyone might be best because you already have a strong army. Remember to keep an eye out for airports. If you have control of two bases with an airport your units can fly between them.
The enemy has three captains that can use cannons. If they use their ability on your base the you'll lose half if your units. Keep this in mind when sending units in groups.  
The only challenging enemies are Natalie and Junka. Using time changing items can make a huge difference in the outcome.


This stage is just like the first stage in the surface world. There are 4 enemy armies and you need to eliminate them all. Make sure you're stocked with items and partners with all of your members. Terraforming and keeping type advantages are key. I like to take down the water area, then win, then either of the remaining two. Some enemies might be worth persuading, but you won't need more than 10 captains.
If you brought one of the cannon captains from last round you might want to bring them to lower the enemy's unit count. After this round you'll make your way to t h next world.
You can find an ally name Sicily in Bavalosk. She can be a valuable partner.