Play the skirmishes. That will help you level up your classes and earn money to buy stuff at the R&D. Once you get your Scouts and Shocktroopers to level 11 they'll have access to grenade launchers and flamethrowers which are mighty handy. Also buy the reports from Ms. Ellet and play the side quests when avaliable. They'll help unlock the main characters hidden potentials.
This is my list of best characters you should recruit regarding class because they get the best P and B potentials once everything is unlocked for them.
Scouts- Alicia, Musaad, Aika, Juno and Wavy
Shocktroopers- Vyse, Alex, Hannes, Lynn, Rosie and Jane
Lancers- Jann, Audrey and Hector
Engineers- Karl, Knute and Dallas
Snipers- Marina, Catherine and Oscar
Welkin's Orders are god-like and many of them comes cheap which is also handy when you get closer to the end. Visit the Cemetery after every battle and buy Orders from the old man when avaliable.