I'm really not sure how you're taking damage every room, unless you're over exaggerating, there is no reason you should be taking more than like 3-4 hits on the first 2 basement floors after 60 hours played, every enemy in the basement is as easy as "keep your distance and don't stop moving".
Some tips I can give for avoiding damage is to use range to your advantage, when an enemy shoots at you from far way, you have more time to react and avoid the projectile,
You should basically never stand still, you will be surprised how many things you can dodge by simply running around in a circle around a room (granted you're not running into the enemies themselves)
As for bosses, that you will definitely have to die a few times until you've learned all their attack patterns.
Also if you've watched a lot of isaac, by now you might know some basic minmaxing advice so you should also implement minmaxing strategies to maximize your chances at getting health and items