When I fight Lost Number (monster in safe) I always take with me; Choco/Mog Seal Evil, Cross Slash. All of those will stun him for a while, and if you've leveled up Choco/Mog you can use it again and again. I'd also use Hypers on your characters so you can get limit breaks quicker. If you keep him stunned, you'll be able to kill him before he changes color.