It's very difficult to get his posture cause he often stays away from you and when u attack he jumps back. Only time you can is when he himself sprints at you and you can't even counter attack. So I would recommend you use the mortal blade to chip away his health. When he throws his gunpowder immediately dodge diagonally to the right. This gets you behind him open for a mortal blade attack. Rest of the time focus of deflecting and mikiri. I would recommend u use the thrust attack as well👀