Doing Case Files And Saving Survivors (Bringing Them Back To The Safe Room) Is The Best (Non-cheesy) Way To Get Experience.
Whenever You Save The Game, Die, Lise, Or Beat The Game -
You Are Asked If You Want To Transfer Your Character From That File To A New File.
This Moves Over Your Level, Combo Cards, Notebook, And Money To The New Game.
Get To Level 50 Across These Playthroughs And The Achievement Is Yours.
Good Luck!