Disgaea 3: Absence of Justice

Only legendary items (flashing gold) have 100 floors. You should designate a character as a runner, probably a moth monster or a masked hero (unlocked with a level 45 gunner and thief) as these classes are not blocked by enemies. You should probably level up their movement speed once or twice in the class world and equip them with 3 pairs of shoes (possibly also leveled in their respective item worlds to each have +1 movement, for a total of 2 per shoe). You will also want a martial artist with at least Triple Strike and Final Arts.

The strategy is to bypass all enemies on each level and go straight to the gate with the runner. If he can't make it, help him out by throwing him. If there is a gatekeeper, throw your martial artist and use the appropriate skill to move the gatekeeper off of the portal.

If you are a low level, you should use a low level item. Since the item world is random, it is possible that on rare occasions you won't be able to move the gatekeeper and if the collective strength of your party can't kill him you're screwed. Also, be wary of mystery gates on higher floors. Some of them can take you into battles with strong enemies. If you win the battle the item will level up, but if you can't, wait two turns and you'll be booted out of the battle.