Monsterland is where you go grind levels between chapters in Mamatoto. The enemies you fight will be the level of Mamatoto, and the types are random.
Every ten times you do Monster Suppression, Gunagan will show up. You can retreat and reset the counter, so don't think you have to fight him at any time. If you do intend to beat him to get the cg he gives, know these facts.
Gunagan will do about 150 physical damage to each person on the field per turn. He has 30,000~HP. His level will go up with Mamatoto's and his attack will go up slightly with his level. Just like the final boss, even though Gunagan is huge he only has one spot where you can hurt him, so limit melee character usage so that they can all do something. You are forced to bring 5 characters into the fight and each times one dies you can replace them. With some grinding it should be shouldn't be too hard, in fact the final boss might be harder.