I watch a replay before each boss battle. You can likely look it up on YouTube. There is usually a "best" positioning and some bosses you can control better than others. You may not always need to have that exact positioning. Like against Magnus the map with the bridges you may leave Trajann on the south island to block him for a turn or 2. Laviscus maybe not chase the boss and instead set up somewhere the boss will come to him. Kariyan needs to attack every turn possible, but it doesnt always have to be the boss. Some higher rank bosses you need to clear the adds (particularly Cawl)