Yes, the Dark Assembly can be a pain in the ass and it doesn’t get much better in Disgaea 2. My best advice would be to start bribing ASAP. Later when you’re much more powerful you can just pass everything by force, much faster and entertaining that way.
As for levelling when needed, use the Item World to level up you weapons and armor. It will help out immensely. Also, levelling gets much easier, if not tedious, once you can open the Cave of Ordeals. One of the maps have enemies set up in a 3X3 square ready to be nuked.