You're going to take damage in the fight against Mothula because of how ridiculous the room is. So accept that and go all out offensively and just hack away with the sword. The only tactic I know of is to go to the top of the screen once Mothula starts shooting rings down. Bring as many fairies as you can and collect as many heart pieces as possible. Also, since ALTTP is nonlinear, there is no shame in leaving Mothula until later. You can go on to the next dungeon and the dungeon after that etc., you don't have to beat Mothula until you want to go to the final dungeon so you can just wait. By then you could have found all the bottles, upgraded your sword, and gained more heart containers; Mothula would be much more manageable then.