For reason or another, there is always a need for greater power in JRPGs like Oninaki. Often times this is obtained through countless hours of play, killing every little foe along the way, ending with you ultimately hitting the cap and having nothing else to do. Some people like to break this cycle, either to avoid less effective grinding in the long run or simply enjoy the story. Whatever your reason might be, Oninaki actually makes it pretty easy to power level.
As you progress there are a number of great daemons that you can use. I strongly suggest using multiple daemons so you can obtain more stones to unlock abilities but if straight levels is your desire, I would advise using Dia. This is for two reasons.
Her first skill, Noble Dream, is a massive forward firing attack that can hit multiple targets. Since you can hit more enemies, you can obviously kill faster to earn more experience. Behind that skill on her tree is a memory, which behind that is called Generosity, an extremely valuable perk. It allows your shots to pierce, giving players the ability to hit multiple targets in a single attack. Obviously, the more things you can hit, the faster you gain experience from fallen enemies.
After a couple hours you’ll find a black hole (pictured above) with an obscene amount of health. These summon enemies to protect it and is an easy way to get tons of experience. As long as you don’t destroy the hole, you can kill enemies for hours. The nice thing is that they’re always grouped up, due to coming from a central location, meaning you can Noble Dream most of them and finish off the rest with shots. Simply rinse and repeat until you get the desired benefit, be it levels, skill upgrades or just more stones.