So, completed the game for the true end, but completion save not possible? I had a screenshot of someone completing the game and it shows a red crystal.
Also, please do the kings quest if you can, because this allows you to unlock the atelier workshop that's beside the shop at the right side of town. Hence able to enhance equipment. (Me as a Dumbass thought everything can easily one-shot you, but no, I just had to the kings quest to finally unlock the feature to enhance my equipment...) Also gargoyles at the 17 floor in groups give more BP. Though this grind spot is a bit of a walk ways. Use high speed shoes to get there fast.
tips to wipe the floor of every mob, DESTROY! skill.
Places to grind, 9th (If I remember, its a group with perverted gentleman and two dogs) and 17th (From the top exit) without getting one shotted. You might also need burn immunity for perverted gentlemen and the dogs to avoid getting burn damage over time.