Sword of the Necromancer

If you find any accessoire (like masks) during a dungeon run that have a 3x EXP bonus, keep it, no matter what other (anti) effects are bound to it. If you have 2 items equiped, you have a x6 multiplier and each bigger enemy nets you 300+ exp, which means leveling up is quite fast, even in the later levels. Of course it depends on luck if you find this kind of item, you could also farm for crafting materials that lets you add the 3x multiplier, in my case i hadn't enough crafting material even at the end of the game, but maybe you are luckier with the drop rate of the needed materials.

Chests can also contain a "level up" effect that instantly raise your level by 1, up to the maximum of 20. So it's worth checking all chests anyway. Good luck!