While Ninety-Nine Nights does fit the formula for a hack-and-slash game pretty well, there are aspects of it that dont work out very well. As it happens, those are the elements that would go best towards the game itself being more enjoyable to play.
In the world of Ninety-Nine Nights, humanity is in a fight for its life against different races, whether lizard men or goblins. You'll play as Inphyy, a girl who's adventuring for vengeance to destroy the goblins who killed a member of her family. There's also a bunch of other characters whose storylines you'll play through in order to reach the end. The idea is a nice one, but because most of the different characters play through the same areas, there is a lot of repetition in clearning those areas again and again.
The game plays just like a formula hack-and-slash game. There are, as mentioned above, two different attack buttons. Switching between them will cause your character to engage in powerful combos that will become more powerful as you gain experience and level up over time. These more powerful combos take time to pull off, though, which will open your character up to being hit while they're preparing to perform to the skill. This means that the most effective combos are usually the simple four-hit combos that you start the game with, and venturing beyond that only makes it more difficult to play and win.
The game also has weapons that you can pick up as you adventure through each area. These don't do much more than change the combat animations that you'll perform. You still do the same moves that you did before you picked up the weapon, but it will visually change how that move is done. It would be nice if there was a more clearly-stated reasoning behind picking up new weapons than just cosmetic. You can also go into battle with allies on your side, which brings a little bit more of an epic feel to the battles than a player would get if they just went into it alone.
The game's mechanics are obviously based on bringing absolutely enormous numbers of enemies onto the battlefield. That's too bad, because the game actually suffers from a level of slowdown when there is a really lot of enemies onscreen at once. Considering without lots of enemies around there's not much purpose to what you're doing, it could be done better, but it does get the job done. There's also the annoying fact that at distances the engine blurs enemies, which doesn't make them look any better, but does help to make you feel like you're going cross-eyed.
While Ninety-Nine Nights does fit the formula for a hack-and-slash game pretty well, there are aspects of it that don't work out very well. As it happens, those are the elements that would go best towards the game itself being more enjoyable to play.