Spam the highest Charisma map you can clear and use the fodder to level units. Repeat until you gain access to Return to Ruins, which is the earliest of the Unit XP grinding maps, the others being Fiery Oasis, Base Scramble 2 and the very last story map currently available whose name escapes me atm. Deep Forest Road is good for gold if you're short, and Remnant Vanquish 3 is a jack-of-all-trades map that gives less unit XP than Return to Ruins and less gold than Deep Forest Road, but is better Prince XP and more gold than RtR and more Unit XP than DFR.

For Prince leveling (to increase Max Charisma, as well as access Iris (Gold Healer) and Cypria (Platinum Rogue)), generally the highest Charisma story map clearable is best. But your focus should be your units first.