This ins't really a direct answer but more just advice.
In general you need to be smart about cultivating your weapons as you go through the game or else you'll hit plateaus (like the one you are at currently).
Each enemy is vulnerable to a specific weapon type, so you'll want to have one "dominant" weapon of each type with you at all times.
To elaborate on what I mean by "dominant", weapons vary in strength tremendously and drops are random. On any given floor you might get two versions of the same weapon with vastly different attack properties. For simplicity's sake, let's just focus on attack power. You might get one weapon with 20 for attack and another with 65 on the same floor. Again, there are a lot of different weapon properties so it will take a bit of experience to decide which weapon types you want to hold on to. To start with I would just look at the attack stat.
You want to make sure that when you are managing your inventory and upgrading weapons that you keep this one dominant weapon of each type in mind. Upgrading weapons doesn't combine/merge stats. You are just destroying one weapon in order to add some EXP to the other weapon. I didn't understand this at first (though it seems obvious now) and I ended up destroying some of my best weapons in order to add a bit of experience to some other marginal weapons.
To summarize, have one dominant weapon of each type with you at all times. When you get a new weapon, try to determine if you are going to use it for combining or if it is a new dominant weapon. If you do this you will always have at least one good weapon of each type and you should have no problem dominating any/all enemies.
