Your description of the situation is pretty shallow, so I will just give some general tips, since I can't know what exactly you're doing wrong.
First of all, your statement about it dealing 100 damage is either an exaggeration, or it means you're playing on master mode. If you're playing master mode - stop, it isn't meant for beginners, you're just gonna get yourself hating the game. Get the best defensive options - iron skin potions, endurance potions, also reforge all your accessories either to warding for more defence, or menacing for more damage, menacing is usually better though. Also, make sure your health is maxed out, and get some other things like campfires, bast statues and heart lanterns.
Second of all, grenades might not be the best choice, since creepers and the brain itself like to be in your face, so you will likely blow yourself up easily. Use other piercing options such as spears or bows with jester arrows instead.
Third, you need knockback for it's second phase. This is the only boss that can get knocked back, and you need to knock it back durning it's second phase, otherwise it will destroy you.
Hope this can at least somewhat help