As someone who has done BL Maddening 3 times, I can say that it gets better definitely as you do more auxiliary maps. Just make sure to have Annette and Mercedes (and other mages like Lysithea) to heal in order to gain HP. Units with low attack like Ingrid or Ashe should finish what others leave out. Sylvain is pretty open in terms of how you want to build him, so just make sure he gets EXP accordingly. Turtling with Byleth, Dedue, and Dimitri is fine. It's normal.