Oswald is a total beast. If you rock him out with Unlimited Power, Painkiller up, pop Overload and then Shadowform (which you get at like the 33-35 range), and equip the +30% attack bangle, man you dice up bosses in seconds.

Without the use of items though, Mercedes is a powerhouse. Her attacks do so much damage, and if she gets in trouble, you can consistently fly away to safety. Her piercing shot does a ton of damage, about as much as Phozon Burst and it travels across half the map sphere I believe and costs half the price (1 phozon bar as opposed to Phozon Burst's 2).