Increase your monster gang!
To summon some new monsters, make use of the incubator you will unlock at a certain point in the game. You should always create new monsters, to have the full set of attack options in enemy hideouts.
The best thing: You can level-up each monster individually and train certains skills to them. Once you merge 2 monsters, the new-born monster will obtain both skills of its ancestors.
The best plan is to train as many monsters as you can and merge them to have an army of strong monsters. While doing so, you will need to create more and more monsters to fill the gaps.
In enemy dungeons, you have equal attack chances to the amount of monsters in your gang. There is a monster limit for each monster leader, which you can increase with the totem, another building in Monster Hustle.