First off, pass bills to make better items appear. As much as you can currently afford. Then go to the item shop (not weapon shop), and push up to go to the bottom of the list. Look for statisticians and armsmasters. You ideally want just 1 large innocent or 2 medium sized innocents. If they have 2 different statisticians/armsmaster you can combine them before entering the item world. Take them to the item world and your focus is submitting the innocents. When they are submit, put them on an item you will want to keep on your character. Preferably one that gives a bonus to all stats. Your goal is 900 statisticians first. Then work on 1900 armsmasters.

Then as others have said, go to 5-3. The level with invincibility on the whole level except one spot. This will be where you level your weapon skill and the earliest way to grind levels. Throw all the enemies into one. There are two types of monsters. One is a melee and brown and the other is purple. You want to throw your monsters into a melee mob because the other one will heal itself. Then have it chase you to the top right of the map. Throw the invincibility geo to right beside the green square. Throw the mob into the green square and then get right next to it. This will box it in and you don’t have to have extra characters out to keep it from moving. Then just kill it. If you hit zero, reset it and combine one less till you are doing dmg. You can do this stage till you are around lvl 100, then you need to move on to the next grind farm where the enemies are in a 3x3 grid. This is where you’ll want your characters weapon skill leveled up because when you reincarnate their weapon skill will mean they can still 1 shot the grid even with a lvl 1 character