Leveling in the early game is so much easier if you unlock the dodge ability asap, also upgrading the broken sword and off-handing it for the XP bonus. (If you go and grab the blacksmith key right away you get a free skill gem and dodge is unlockable without any sacrificing.) You can take on some pretty tough dungeon bosses while under-leveled if you just learn their patterns, dodge while chipping away. Defeat and recruit Bunshido as soon as you can, it's a bit tough to do early on but she is insanely OP and will make mince meat of everything.
Alternatively, here's something cheatier:
Defeat the giant rat at floor 5 of the sewer dungeon so that they appear in town. Shouldn't be too hard if you abuse potion buffs and Size Explosion. Recruit them, confiscate their weapon, then dismiss them. Now you can challenge them to easy boss fights as many times as you want, earning thousands of XP with each win. This also works with barrel master, chimera, bunshido and foreman, so pick whichever you're best at fighting