Step 1: Pick up some powerful friends from the Friends List topics on this board
Step 2: Use these powerful helper squads to carry u thru the story maps (don't go for 3*, just clear the maps)
Step 3: Spam the hell out of 12-1/12-3, using the SG spawns to get level 20/100 manyus to powerlevel your girls (make sure after the mission cost you will have 80 stamina to actually run the SG since it expires after 5 minutes)
Side Note: Powerlevel your single highest rarity unit because your helper squad's stats actually get scaled down to the level of your strongest girl, so powerleveling a single girl will give you more power from your helper squads