This is one of the best games I've ever played. It has a dark souls vibe where you begin with nothing and it's very hard, but as long as you progress only a little each time, you will get tokens which help you level up your hub world, making future runs a little more easy.
Not only that but there are many reasons to keep playing. I've put in more than 100 hours and still haven't completed the game. There are over 100's guns and the same amount of items and other things to make your runs harder or easier. You never really know what you will get. You might get the best guns on a run with the best items and power ups which make each boss a cake walk, or you might not. However none of these things means you won't be able to beat a specific boss.
There is a main boss which is a dragon, but once you beat him it's only the beginning. You get certain things along the way which help you build a thing called "bullet from the past", this will allow you to take on your characters past once you gain the bullet and beat the dragon, however getting to the dragon and beating it are never guaranteed!
There are other characters you can get to play with pasts to beat, but you need to do certain things to get them. Then there is the rat (who I still haven't beaten). He takes items left in a room that you leave, but if you do the right things you can get to take on the rat, however there are a lot of moving parts to do this. There is also another dungeon to take on which I haven't really taken much time to do. Then after all this, there is the last boss who will grab you if you step into a particular area after beating the dragon.
I love this game, it proves you don't need flashy graphics to be a hit. I'm still playing it today, I'm on a good run with great guns and trying to beat the bullets past. Each time you finish a level you can save for another day. So I'm taking my time anf tonight will finish the last level, beat the dragon and the bullets past!