Mr.Tommasi is the hardest boss in the game because he is very accurate at hitting you and it deals a lot of damage to you, also there are a lot hisses, and there are a few Invisable hisses. Once the fight started walk up the stairs and try to reach the very top of the room where there is a walkway that spans across the room. From there Mr Tommasi doesn't fly above you most of the time. (You will probably die if he did, if he did then you will just have to try again). After you get rid of half his health bar, everytime you deal with the flying hiss, the invisible hisses appear. Look for them as they come from the either ends of the walkway, kill the invisible hisses before they gets close and explode. Now there's some spare time before the flying hisses respawn and you'll have time to deal damage to Mr.Tommasi. Health is very limited in this fight, the only way to regenerate a lot of health is to collect the health dropped after killing an invisible hiss.