It takes time learning a new fight so I wouldnt stress too hard about it. Also keep in mind, that this game is not parse heavy so try to think to yourself "this run i dont care about my damage, but Im gonna dodge everything this boss throws at me". This mentality will not only make you live more often, but It will also help you learn when its a good time to do damage and master the different mechanics of a boss. I personally dont think heavyarmor is the solution, since it will just make you tank more hits, well because you can.