Ok the easiest way to level the characters you want to level 20, say Seth Franz ,Eirika, Artur and whoever else you may choose.... when you start chapter 5 kill everyone or if you would like Get Joshua to join you then kill everyone but the boss, Now there is a armory and vendor here for your convenience, easiest thing to do is have a healer with you with a lot of healing staffs.... Now there is an arena in the area that you can constantly use to level after everyone is dead but the boss, be careful use your save states wisely especially when trying to do the arena with Eirika, if you lose in the arena you lose that character permanently or in Eirika's case Game Over. Always use save state before going into the arena because you never know what they will throw at you, after about 30 minutes I had a majority of my party with me at the time at level 20, use your healer to constantly heal the person your leveling for them to gain experience as well. Hope this helps the people who are having a hard time with this game. Plus by the time you level your party you'll have close to 50k in gold