To level your weapons faster, go to the level Terrible Cold in the area Jotunheim (it should be the one with invincibility on all the panels except for one). Make sure you have a character with a high amount of counters(the ninja class or the majin class) a strong person who could kill a level 100 person, and the person you want to master the weapon. First, start by throwing all the enemies into each other. Then when they are all combined, have someone pick the enemy that you just made and hold em. Next, have the ninja or the majin and the person you want to level up come out and have them beat each other up. If you did it right, your team members should be beating each other up with regular and counter attacks. Just do this for as long as you want. Each hit raises their weapon mastery. To speed up the process you could go to the item world and get the armsmaster specialists and attach them to a weapon, so your character levels faster. When you are done levelling, kill the enemy that you were holding.