I spent forever trying to beat this boss, ended up switching weapons, and trying different tactics. I suggest practicing on phase 1 until you can do it with minimal heals without burning many consumables / throwables, so that you go into phase 2 prepared. Personally, I never did get to the point that I could consistently stay away from the boss, so this run was a bit of a fluke and I almost died again. Until you get him under about 50% with some heals left, you probably don't want to be burning all your throwables, unless you made a backup save to reload every few attempts, otherwise you'll probably end up having to farm more as this boss can be a real wall to finishing the game.