This will work best after you have completed the first play through, since you will acquire the Demon Force Tactic in the last boss fight...
You can level up and most like never have to engage the enemies in the field in direct combat..
Thats what I did, choose the Open Area Room and use the following Decors:
Silent Walk - You will not get targeted for 3 turns Secret Guide - Stats down 50%, but you get 5x EXP [when the room acquire the 25 stars you will get 10x EXP] Burning Blood - Number of special and field Tactics +2 [again when the room get the 25 stars it will go up to +7] And for the 4th decor you can use either Paw Day or Vengeful Hope [if taking just the Hero] to compensate for the Stats loss from Secret Guide..
I did use Human Power [if unit is Human, stats up 20% (when the room gets the 25 stars you will get Stats up 45%)], and did carry a full party, since there will be times that you will not be able to finish the Room Sentry or Tyrant before Silent Walk expires [you will come across the Gypsy enemy or an Antimatter room].
I did 65 stages in less than 2 hours and level up from 49 to 126 and only engage in direct combat 2 times. When you fell comfortable just start doing Inspections on higher level rooms using this same technique.