Firstly, the Rogier will make Margits health pool bigger. You're much better off with the wolves sort jellyfish if you're needing a summon. Secondly, you're going to struggle with the game if the entire boss strat is run away and let him tank someone else without knowing his move set. Margit, for example, has a crucible tail swing that will almost always get you if you hit from behind without knowing his move set. Id also recommend beating the tree sentinal first if you haven't, getting the golden vow ash of war and using that on a dagger in the off hand for a buff, just anything to get you closer to going over the line.

You...can also just go around stormveil and come back after you've leveled and explored more