This is an open-ended question with a lot of different answers to it. For one, it really comes down to what game you are asking about. If you are asking about SMT III where his debut happened, it comes down to your player level and how much you have upgraded his health. For players looking to get higher health points, you need to level up and then apply attribute points towards the endurance stat. This will upgrade your health. You can get it quite high, especially if you max out the stat to 99 and then start to add HP-increasing accessories into your build. As for the other games where Demi Fiend appears as a boss, this varies, too. In the Imagine MMO, his guest appearance had a whopping 10 million health to battle through. At the same time, though, it was only a measly 1200 in SMTIV: Apocalypse. For those of you who are playing the Digital Devil Saga, though, you are looking at about 18,000 health.