100+ hours of gameplay AND powerleveling to 9999? SIGN ME UP BABY! I HAVE NO LIFE TO GIVE TO OTHER THINGS!
There are so many ways of leveling up to infinity and beyond that I can't even hope to list, there is so much depth to the game, but you can beat it with minimal knowledge, and never even attempting to gain access to extra netherworlds.
There is even the Dark Asembly to look into, that is where you get all your little extras passed, such as reincarnating characters to level 1 and making them stronger after a while, or even making it easier to get a higher bonus level at the beginning of battle, there is just so much to do and too much life to get in the way of.
Even though it takes hundreds of hours to make the ultimate characters or infinity+1 equipment and items, there is a lot to be had and if you love J-RPGs like FFTA or Fire Emblem, you should pick up this game and start laharl on his way to being a bad ass freaking overlord.