Make sure to get some strong allies, either by the forum thread, or by checking the bottom of the helper squad list before every battle. Once in a while, there will be a much more powerful ally available. If you keep asking, you will get some eventually. That's what I did, before I found these forums.

Then, instead of trying to evenly level up your squads, try to max one of them out. Remember that by having one girl at level 60, and later 70 post evolution, you will maximize the power of your helper squad. This will give you two powerful squads that should help you complete 80 stamina maps.